If you want to run a ton of VMs on a server, Xen is great. It's fast and stable once you sort through the mess of getting a kernel that both supports your hardware and runs well as a Dom0 (the "host" machine).
And for the past year or two, most distros have shipped kernels which would boot just fine "out of the box" on Xen virtual machines.
It's what most "cloud" or VPS providers run (including Amazon, Rackspace, Slicehost, Linode, etc).
However, if you're running a desktop and want to virtualize, Xen is probably going to be a HUGE pain in the balls, with no real benefits. Just use KVM or VirtualBox.