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They expose a virtualization API that allows this on Mac App Store: http://veertu.com/

Don't know how robust it is compared to e.g. VMWare or VirtualBox, but it seems to be working well enough in my simple tests.

That same Hypervisor.framework is also what powers https://github.com/mist64/xhyve

I was really excited about that project but it doesn't appear to be getting much attention.

Probably run Linux. Xserve is dead, I don't think Apple uses OS X on servers.

iCloud uses Linux according to their job postings.

Linux on non-apple machines.

HP machines to be specific.