XenClient: a client side hypervisor

I'm sure we all know about virtualization and hypervisors, but what about running virtual machines on your pc / laptop? I'm sure we all have used Vmware, VirtualBox, VirtualPC or Parallels in the past to run multiple virtual machines on your machine. But these all require a parent operating system. For the most part, testing different operating systems / having seperate environments for specific projects work well in just such an environment.

But what if you want to have two completely separate environments running, that have no way of even knowing the other exists?