Few years ago, I'd have ranked VMworkstation 12 first, KVM/QEMU next and then VirtualBox in terms of sheer speed. Didn't seem to be much between them in terms of functionality (until you get into esoterica like importing a host PCI device into the guest).

Seem to recall there was ~20% speed difference between VMware/KVM, but a bigger gap (maybe 30, 40%), between KVM & VirtualBox. Just based on casual observation (side-by-side Debian installers).

oVirt was also attractive - except that it's joined at the hip to the Fedora/Red Hat universe and their clones. Being tied to an LTS kernel/libc model is just too restrictive for also running bleeding edge/fast-moving software. If you don't need to track these, it can be an option too.

Running KVM under libvirt adds an abstraction layer and significant flexibility - like GlusterFS/ZFS integration for storage pool(s). It'll also manage/run LXC containers from the same interface/API. Add virtual networking (Openvswitch) on the host, and this is pretty useful.

Trying to do anything comparable on a 'Doze host? "that'll end in tears"...