The people you refer to as Xen are probably XenSource. Xen itself is a hypervisor, tool set and kernel patch, XenSource is a company that sells a Xen based Linux product (that smells like CentOS) with some very, VERY polished tools. Also check out Virtual Iron, who sells a different spin on Xen. Their concept used to be that you plugged a CD in one machine and let the rest of the machines network/PXE boot; smooth but not what I needed so I haven't looked lately. Red Hat Enterprise 5 has Xen tools built in. I'd say it IS being supported, and since you can count on a minimum of 5 year support for any Enterprise edition, (probably more like 10 to 15) I'd say that Xen won the Red Hat nomination. There are some other serious players on the field, but the battle seems to be boiling down to Xen Vs. VMware. Xen is cost free, free to download, open source and you can get it with CentOS 5, Novell, Red Hat Enterprise 5, Gentoo, Debian or probably through any other major Linux distributor now. VMWare is not open source and probably never will be. VMWare is owned by EMC who has lately not been at the top of the charts for customer satisfaction. It is free of cost, but if you look at Enterprise edition cost comparisons you need a VERY hefty budget to consider VMWare. Of course, every distribution that sells support will tell you that they have the best one. Ask any of the distros using Xen and they'll tell you why Xen is better than VMWare, ask EMC and they'll tell you why you need VMWare. Shocking isn't it? The really interesting news is that Window's next server edition will almost certainly use the same same type of virtualization that Xen uses. Longhorn (this incarnation) plays nice with Xen on Novell and Longhorn hypervisor hosts Novell nicely too. Short version: * Microsoft, Red Hat, Oracle, Novell, and Debian all seem to be planning on working with , or flatly supporting Xen. * Xen was designed to take advantage of new hardware options from AMD and Intel (new enough that you still have to ask specifically if you're buying servers) * EMC's VMWare offers an older hardware the ability to do virtualization that Xen cannot. Did you know you can run VMWare inside of a Xen based system? I don't know about efficiency, but when I tried it, it worked. »