When they say OS requires modification, what do they mean in Virtualization?

Question

I'm reading an article about Xen, a virtual machine monitor. They say that an operating system requires some modification in order to be able to act as a guest OS on top of Xen. Now, for an OS like Linux, I can understand what a "modification" might mean but in the case of an OS like say, Windows XP, what does it mean? I mean, XP is closed source proprietary OS right?

Answer 1

Xen was originally a university project. As a researcher (or as a government agency tasked with infrastructure security), you can get the Windows sourcecode from Microsoft. You're just not allowed to distribute your own version of Windows.

This is exactly what they did: they had the sourcecode of Windows XP and then they ported XP to Xen, just to show that if Microsoft were ever to officially support Xen, it would be doable.

Answer 2

It means exactly the same thing. It's just harder because the source is not widely available. Note that the modifications are no longer required when Xen is used in conjunction with hardware virtualization.

Answer 3

On the other hand, although para-virtualization cannot provide an end-to-end virtualization solution, it can improve performance.