Virtual Iron is also based on Xen, but has decided against paravirtualization and offers "native virtualization" using hardware-assisted virtualization technology -- limiting the possible servers that can run Virtual Iron. This may or may not be a problem for you depending on your environment.
Virtuozzo forgoes server virtualization altogether, in favor of OS Virtualization. With OS virtualization the operating system is partitioned instead of virtualizing a whole computer. This means that it is not possible to run mixed operating systems on the same host. You must run Linux on Linux and Windows on Windows.
Overall, I believe that Xensource and Virtual Iron offer the most flexibility of the three. However, if you are standardizing on one operating system anyway, Virtuozzo may be an attractive alternative. Either way, competition will be good for the whole virtualization market.
This was first published in January 2007