[HOWTO] Ubuntu Lucid 10.04 LTS on XenServer in PV mode
Installing Ubuntu on Citrix XenServer is fine; but to get better performance it should be running paravirtualised (PV) instead of hardware virtualised (HVM - the default). Ubuntu isn't natively supported, so it needs a bit of manual work to achieve it.
I've put together a page on our wiki that details the steps needed to get it up and running in PV mode and get the XenServer tools installed (and running at start-up!).
Ubuntu 10.04 LTS paravirtualised on Citrix XenServer [Bishop Barrington IT Wiki]
If you spot any mistakes please correct me.