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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Virtualization is a technology space where the platform rules. This explains why our top certifications originate from vendors who offer their own specific virtualization platforms and tools. These leading companies include VMware, Cisco, Oracle, Hewlett Packard Enterprise and Microsoft. (Citrix and Red Hat also offer virtualization certifications for their platforms, and are mentioned at the end of this article.)