Previously i mentioned that hosting was on a Xen virtual machine. This is incorrect. It is actually accomplished with a Virtuozzo virtual machine. Admittedly the outbreak of virtualization technologies is making it harder to keep track of all the solutions available.

Virtuozzo is implemented at a slightly higher level than Xen. Xen exposes an API for modified operating system kernels to use to perform their relevant tasks. Virtuozzo on the other hand, uses just one kernel which partitions resources and acts like multiple machines.

Which is better? Well that would depend on who you ask.

Xen offers more choice allowing for things like running BSD under Linux.
Virtuozzo in theory would have less virtualization overhead, and possibly better overall management of resources.

For further reading I suggest you start out at Wikipedia – Virtualization.