XAPI Ubuntu Install
Ubuntu is supported both as a PV or HVM guest on XCP. Ubuntu 10.04 LTS works out-of-the-box as a PV guest.
There's some helpful information on the Ubuntu wiki, but bear in mind this information is oriented towards a local Xen configuration, not specifically XCP:
The easiest way to install a new Ubuntu guest is to use Citrix XenCenter and one of the built-in Ubuntu templates.
If using the Ubuntu template in XenCenter to create a new VM, enter this URL as the InstallURL to setup a new copy of Ubuntu: http://archive.ubuntu.net/ubuntu. This will allow you to easily do a network install with 10.04.
If you have boot problems, please read XCP_Ubuntu_PV for more information about configuring the Ubuntu kernel to boot correctly from XCP.