KVM seems to have the most support "nowadays" so if you are starting out or looking what to consolidate on it's probably the best choice of the two unless you are looking at density, in which case the para-virtualised Xen is better.
We always recommend Xen virtualization because it is fully virtualization on systems that support virtualization extensions, but can also work as a hypervisor on machines that don't have the virtualization extensions. Xen is a hypervisor that supports x86, x86_64, Itanium, and ARM architectures, and can run Linux, Windows, Solaris, and freeBSD
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