Intenral6 is the start of an attempt at replacing a bunch of ORBIT infrastructure with a few hosts that run virtual machines. ORBIT uses multiple machines because of load concerns, but it's hard to imagine more than one or two machines loaded at the same time.

Hardy (Ubuntu 8.04 LTS) comes with a full compliment of Xen software, it's just unsupported.

$ sudo apt-get install linux-xen xen-hypervisor-3.2 xen-shell xen-tools xen-utils-3.2 bridge-utils

Somewhat confusingly, the linux-image-xen packages don't put turds in /boot/grub/menu.lst, rather the xen-hypervisor package does. The linux-image-xen packages are sort of the payload for the hypervisor.