This raw full disk image is a straight-up install of Ubuntu 12.04.1 Server LTS "Precise Pangolin" on EXT3. No packages were selected for installation other than OpenSSH Server. It will boot under full virtualization platforms (QEMU, VirtualBox, VMWare) without modification (except maybe disk image conversion) but the steps below are required to make it play nicely as a Xen Paravirtualized Guest.

db3fb8b154cb05ab704733bba1a6d70e ubuntu-12.04.1-x86_64.raw.hdd.lzma

The user account is "user"
The user password is "user"

The default network settings are: eth0 DHCP

Fortunately, the generic kernel has Xen PV guest support so the required buggery is mostly limited to making the console work. Unpack and mount the image:

# unlzma ubuntu-12.04.1-x86_64.raw.hdd.lzma
# mkdir /mnt/rawroot
# lomount -diskimage ubuntu-12.04.1-x86_64.raw.hdd -partition 1 /mnt/rawroot

The distributed /boot/grub/grub.cfg script will make pygrub throw up. We'll drop this (/mnt/rawroot)/boot/grub/grub.conf file in to override it:

default 0
timeout 5
fallback 1

title Ubuntu, with Linux 3.2.0-29-generic
        root=(hd0,0)
        kernel   /boot/vmlinuz-3.2.0-29-generic root=UUID=a52e9498-44e8-4c0d-807e-903d4e19e204 ro console=hvc0
        initrd  /boot/initrd.img-3.2.0-29-generic

title Ubuntu, with Linux 3.2.0-29-generic (recovery mode)
        root=(hd0,0)
        kernel   /boot/vmlinuz-3.2.0-29-generic root=UUID=a52e9498-44e8-4c0d-807e-903d4e19e204 ro recovery nomodeset
        initrd  /boot/initrd.img-3.2.0-29-generic

Now we need to make a getty load on hvc0, create (/mnt/rawroot)/etc/init/hvc0.conf

# hvc0 - getty
#
# This service maintains a getty on hvc0 from the point the system is
# started until it is shut down again.

start on stopped rc or RUNLEVEL=[2345]
stop on runlevel [!2345]

respawn
exec /sbin/getty -L 115200 hvc0 vt102

We probably want to give our VM a static IP. Edit (/mnt/rawroot)/etc/network/interfaces to reflect:

# The primary network interface
auto eth0
iface eth0 inet static
address 192.168.1.100
netmask 255.255.255.0
gateway 192.168.1.1
dns-nameservers 8.8.8.8

Now we are ready to boot up. Unmount /mnt/rawroot and create a configuration file that looks similar to:

name = "ubuntu"

vcpus = 1
memory = 256
vif = [ 'mac=00:16:3e:ff:00:01,bridge=extbr0' ]
disk = ['file:/xen/ubuntu/ubuntu.hdd,sda,w']

bootloader = "/usr/bin/pygrub"
extra = "xencons=hvc0 console=hvc0"

Ensure the MAC address does not conflict with any existing virtual machines. Assuming the configuration file is named ubuntu.conf run

# xm create ubuntu.conf -c

If everything went well you should be looking at a login prompt shortly after.

Note that if you use cp -ax to transfer the filesystem's contents to a larger image in the future, references using UUIDs in fstab and grub.conf will have to be changed to either reflect the new UUID or the corresponding /dev node (probably xvda1) and GRUB must be re-installed to the MBR.

If you would like to move the contents to a filesystem-only image (no partition table, MBR, etc.) which is much easier to grow than a full disk image you can drop pygrub and externalize the kernel and initrd images then reference them in the configuration file.