Short for Kernel-based Virtual Machine, KVM, is not as widely deployed as other open source hypervisors, but its stature is growing rapidly. KVM is a full virtualization hypervisor and can run both Windows and Linux guests.
Kernel-based Virtual Machine, KVM
Xen supports paravirtualization and "hardware assisted" virtualization for modified and un-modified guests, respectively.
OpenVZ is container-based virtualization for Linux which has become quite popular among the mass-market Linux hosting providers as an inexpensive way to provide virtual private servers.
VirtualBox runs on Windows, Linux, Solaris and Mac OS X and can support all those operating systems as guests.