Xen is also used on the desktop, e.g. Qubes (Type-1) or Bromium uXen (Type-2).
Is bromium out of vaporware stage yet? They make bold claims, but I've never heard of anyone actually getting a hold of their tech. I've certainly not seen any reviews by serious security professionals picking apart their offering.
Qubes is awesome though. It really does not get as much attention as it deserves.
Some companies are listed at http://www.bromium.com/customers.html. No sign of third-party reviews, sadly.
Xen is meant for running a potentially large number of server VMs headless. VirtualBox is meant for running desktop VMs. You could make VirtualBox run headless (exposing a pseudo-screen over VRDP) to do what Xen does, but... eww.