The bhyve hypervisor is a nice addition to a FreeBSD developer's toolbox. Guests can be used both to develop new features and to test merges to stable branches. The hypervisor has a wide variety of uses beyond developing FreeBSD as well.

bhyve is a hypervisor/virtual machine manager developed on FreeBSD.

xhyve is port of bhyve to OS X. It is built on top of Hypervisor.framework in OS X 10.10 Yosemite and higher, runs entirely in userspace, and has no other dependencies.