What's this xen then?
I seriously considered Citrix's XenServer earlier in the year for a server platform for a new start-up. I had it installed but then it told me I needed to get a license (which was free of course) but 2 hours later having searched Google and Citrix's own website, I couldn't find the page I needed to register.
I did eventually find it, then noticed the license was only for a year and I thought "what if in a year's time Citrix stop renewing free licenses" - you wouldn't be able to start (or restart) any virtual machines if the license had expired.
So I pulled the plug, downloaded Ubuntu and installed KVM - I've never looked back. The platform is still in test for storage and live migration, but the 4 machines that are running on it as test beds are perfectly stable.
I'm glad I ditched Xen - this article just shows what faith Citrix have when they build something new and exciting and don't put it into their own product first.