This installation process demonstrates how to set up Xen on Debian 8. It doesn't go into great detail and only gives a general overview.

We may assume that this process has not changed dramatically for newer versions of Debian.

We first install Debian 8 with an official CD image and then remove and replace systemd after installation.

NB Replacing systemd on newer versions of Debian is not possible.

Boot the image to perform the installation process and select `Install'.

From the language selection pages I chose the following.

English => United Kingdom => British English

After loading components from the CD, we enter some basic but essential host information.

hostname: xen
domain name: example.com
root password: XXX
full name of new user: ABC XYZ
username: ABC
password: XXX

The next step is partitioning. Initially we choose Guided partitioning and after edit the automatic choice for the root partition.

If you prefer you can choose Manual partitioning. In manual mode you may set up software RAID before creating a volume group and the volumes within it.

Guided Partitioning - use entire disk and set up LVM
  All files in one partition (recommended for new users)
     Write the changes to disks and configure LVM? YES

Do not select finish just yet, go up the menu.

Configure the Logical Volume Manager
  Write the changes to disks and configure LVM? YES
     Delete Logical Volume
       root
     Create Logial Volume
       xen-vg
         Logical Volume Name
           root
         Logical Volume Size
           8GB
     Finish

Inspect the partition list and select the root entry created above.

LVM VG xen-vg, LV root - 8.0 GB Linux device-mapper (linear)
  #1
     Use-as: Ext4 journaling file system
     Mount point: /
     Mount options: [x] noatime
     Label: root
     Reserved Blocks: 1%
     Typical usage: standard
     
     Done setting up the partition

Finally, we finish creating the disk layout.

Finish partitioning and write changes to disk
  Write the changes to disks? YES

After some time installing the base system, the package selection process will begin.

First select a mirror, I chose UK.

United Kingdom => ftp.uk.debian.org
  Continue

Wait for some time.

Participate in the package usage Survey? NO

Finally we reach the software selection menu.

It's important to deselect every entry using the space bar and only select ssh server.

[*] SSH server
  Continue

Once installation is done the boot loader is installed and we are finished.

Install the GRUB boot loader to the master boot record? YES

Remove the ejected CD.

Installation complete
  Continue

Login, update the system and then reboot it.

apt-get update
apt-get dist-upgrade
reboot

Login and replace systemd with sysvinit.

This process may also be applied to any virtual machines other than domain 0.

apt-get install sysvinit-core sysvinit sysvinit-utils

If this is a VM, configure the following.

1. sshd root login (if required)
2. inittab getty hvc0 (1:2345:respawn:/sbin/getty 38400 hvc0)

Now reboot, login again and finish the job off.

apt-get remove --purge --auto-remove systemd
/bin/echo -e 'Package: systemd\nPin: origin ""\nPin-Priority: -1' > /etc/apt/preferences.d/systemd
/bin/echo -e '\nPackage: *systemd*\nPin: origin ""\nPin-Priority: -1' >> /etc/apt/preferences.d/systemd
/bin/echo -e '\nPackage: systemd:amd64\nPin: origin ""\nPin-Priority: -1' >> /etc/apt/preferences.d/systemd
/bin/echo -e '\nPackage: systemd:i386\nPin: origin ""\nPin-Priority: -1' >> /etc/apt/preferences.d/systemd

Install Xen kernel and tools.

apt-get install --no-install-recommends xen-linux-system xen-tools lvm2 bridge-utils
dpkg-divert --divert /etc/grub.d/08_linux_xen --rename /etc/grub.d/20_linux_xen
echo 'GRUB_CMDLINE_XEN="dom0_mem=512M"' >> /etc/default/grub
update-grub

Here is an example set up for an Ethernet bridge.

Edit the interface file.

vi /etc/network/interfaces

Change the IP addresses for your host and network.

auto lo
iface lo inet loopback

auto eth0
iface eth0 inet manual

auto xenbr0
iface xenbr0 inet static
        bridge_ports eth0
        address X.X.X.X
        netmask 255.255.255.X
        gateway X.X.X.X

Finally, reboot into Xen.

reboot

Using the Xen tools we create a Debian Wheezy virtual machine with 1GB RAM and 16GB disk space.

xen-create-image --cache=yes --fs=ext4 --memory=1GB --size=16G --swap=1G --vcpus=1 --gateway=X.X.X.X --ip=X.X.X.X --netmask=255.255.255.X --hostname=vm0 --lvm=xen-vg --dist=wheezy

The creation process will take some time, outputting the following.

General Information
--------------------
Hostname       :  vm0
Distribution   :  wheezy
Mirror         :  http://ftp.uk.debian.org/debian/
Partitions     :  swap            1G    (swap)
                  /               16G   (ext4)
Image type     :  full
Memory size    :  1GB
Kernel path    :  /boot/vmlinuz-3.16.0-4-amd64
Initrd path    :  /boot/initrd.img-3.16.0-4-amd64

Networking Information
----------------------
IP Address 1   : X.X.X.X [MAC: XX:XX:XX:XX:XX:XX]
Netmask        : 255.255.255.X
Gateway        : X.X.X.X


Creating swap on /dev/xen-vg/vm0-swap
Done

Creating ext4 filesystem on /dev/xen-vg/vm0-disk
Done
Installation method: debootstrap

Running hooks
Done

No role scripts were specified.  Skipping

Creating Xen configuration file
Done

No role scripts were specified.  Skipping
Setting up root password
Generating a password for the new guest.
All done


Logfile produced at:
         /var/log/xen-tools/vm0.log

Installation Summary
---------------------
Hostname        :  vm0
Distribution    :  wheezy
MAC Address     :  XX:XX:XX:XX:XX:XX
IP Address(es)  :  X.X.X.X 
RSA Fingerprint :  XX:XX:XX:XX:XX:XX:XX:XX:XX:XX:XX:XX:XX:XX:XX:XX
Root Password   :  XXXXXXXXXXXXXXXXXXXXXXX

List VM images.

xen-list-images
Name: vm0
Memory: 1024 MB
IP: X.X.X.X

Run VM with a console.

xl create -c /etc/xen/vm0.cfg 

Run a VM silently without a console.

xl create -q /etc/xen/vm0.cfg

Attach a console to a running VM.

xl console vm0

List VMs.

xl list
Name                                        ID   Mem VCPUs      State   Time(s)
Domain-0                                     0   512     2     r-----     313.4
vm0                                          2  1024     1     -b----       5.5

Shut down VM.

xl shutdown vm0

Auto-start a VM at boot time.

mkdir -p /etc/xen/auto
ln /etc/xen/vm0.cfg /etc/xen/auto

If we shut down a VM we can perform disk maintenance on its virtual disk from Domain 0.

First ensure that the target VM is off-line. Here we only have 1 VM so our output only shows Domain 0 and our target vm-0 is not running.

xl list
Name                                        ID   Mem VCPUs      State   Time(s)
Domain-0  

Now inspect the volumes for our target.

lvs | grep vm0
  vm0-disk xen-vg -wi-a----- 16.00g
  vm0-swap xen-vg -wi-a-----  1.00g

We will increase the size of the virtual disk and leave swap alone.

lvextend --size 24G /dev/xen-vg/vm0-disk
  Size of logical volume xen-vg/vm0-disk changed from 16.00 GiB (4096 extents) to 24.00 GiB (6144 extents).
  Logical volume vm0-disk successfully resized
lvs | grep vm0
  vm0-disk xen-vg -wi-a----- 24.00g                                                    
  vm0-swap xen-vg -wi-a-----  1.00g  

We must extend the size of the file system contained in the volume.

First check the disk for errors.

fsck -fy /dev/xen-vg/vm0-disk 
fsck from util-linux 2.25.2
e2fsck 1.42.12 (29-Aug-2014)
Pass 1: Checking inodes, blocks, and sizes
Pass 2: Checking directory structure
Pass 3: Checking directory connectivity
Pass 4: Checking reference counts
Pass 5: Checking group summary information
/dev/mapper/xen--vg-vm0--disk: 15147/1048576 files (0.4% non-contiguous), 226267/4194304 blocks

And now resize it.

resize2fs /dev/xen-vg/vm0-disk 
resize2fs 1.42.12 (29-Aug-2014)
Resizing the filesystem on /dev/xen-vg/vm0-disk to 6291456 (4k) blocks.
The filesystem on /dev/xen-vg/vm0-disk is now 6291456 (4k) blocks long.

The VM can now be restarted with an increased disk capacity.

root@vm0:~# df -h /
Filesystem      Size  Used Avail Use% Mounted on
/dev/xvda2       24G  499M   22G   3% /

We may discover that a choice for a swap partition size was incorrect so we may resize it.

In this example we will resize the Domain 0 swap partition but we can also resize the swap partition of a VM in a similar fashion.

First we take the swap partition off-line.

cat /proc/swaps 
Filename                                Type            Size    Used    Priority
/dev/dm-1                               partition       6361084 0       -1
swapoff -a

Now we inspect the volumes.

lvs
  LV       VG     Attr       LSize  Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert
  root     xen-vg -wi-ao----  7.45g
  swap_1   xen-vg -wi-a-----  6.07g
  vm0-disk xen-vg -wi-a----- 16.00g
  vm0-swap xen-vg -wi-a-----  1.00g

We can reduce the size of an off-line swap partition with no dangerous side-effects, this wouldn't be true if it were a data partition.

lvreduce --size 2G /dev/xen-vg/swap_1 
  WARNING: Reducing active logical volume to 2.00 GiB
  THIS MAY DESTROY YOUR DATA (filesystem etc.)
Do you really want to reduce swap_1? [y/n]: y
  Size of logical volume xen-vg/swap_1 changed from 6.07 GiB (1553 extents) to 2.00 GiB (512 extents).
  Logical volume swap_1 successfully resized

We can also increase the size, for example, if the above step was incorrect.

lvresize --size 5G /dev/xen-vg/swap_1
  Size of logical volume xen-vg/swap_1 changed from 2.00 GiB (512 extents) to 5.00 GiB (1280 extents).
  Logical volume swap_1 successfully resized

Now we rebuild the swap.

mkswap /dev/xen-vg/swap_1 
mkswap: /dev/xen-vg/swap_1: warning: wiping old swap signature.
Setting up swapspace version 1, size = 2097148 KiB
no label, UUID=3ac8493d-4dee-4309-9a01-953560ede126

And remount it.

swapon -a
cat /proc/swaps 
Filename                                Type            Size    Used    Priority
/dev/dm-1                               partition       2097148 0       -1