What is VPS?
What is Virtualization?
VPS Technology

XEN The most important Virtualization technology currently is XEN. The primary advantages of XEN are low overheads, and complete flexibility. By utilizing the latest intel VT chipset, XEN will be able to run any guest OS without modifications, and the para virtualization approach results in the least amount of overheads, making the ideal choice in a production environments who want to run a mixed system of Windows and Linux.

OPENVZ the other open-source technology, is easy to manage, and ideal for creating light weight partitions that each customer can manage themselves. The limitation here is that OPENVZ virtualization is done at the OS level, and thus you can only partition linux into multiple linux boxes, and thus prevents you from running anything other than the host OS, that is in this case Linux. OPENVZ partitions are not completely isolated, and the entire ram + swap is shared across the vpses. This would mean that a graceful degradation of services cannot be achieved, but this is more than offset by the ease of adminstration, and the low overheads.

MS Virtual Server is Microsoft's foray into the virtualization space, and creates sand boxed virtual machines in a windows environment. This is ideal of people who prefer Windows as the primary OS, and would not want to install Linux on their servers. The virtualization here is a bit more complex, and thus will incur higher overheads.

HyperVM's final aim is to allow the customer to manage all of these technologies in a seamless manner, and centrally manage these varied virtualization technologies running on different platforms managing different operating systems, all from a single console.

Using XEN technology Windows and Linux OS can be run on same hardware node.