Dear treotan:
Fedora has built in Xen support and thus, patched Xen kernels available. I am not too sure about Ubuntu in terms of installs but what you want shouldn't be a 'big deal' - make sure you have downloaded vnc viewer, if you don't already have it and then try virt-install (...if it is running on Ubuntu which it probably is!) If you don't have it you can get it at:
http://virt-manager.et.redhat.com/index.html
I strongly recommend using the latest version!
It will want to have you get Fedora off the 'net or over a network it does not accept disk based installs, right now.
So...go to:
http://download.fedora.redhat.com/pub/…
...where the "..." is surf their pages to fill in the rest of the string to get the latest version Fedora 8.
Follow the interactive commands for virt-install and it will manage everything!
Best,
Pyrrhic