The xen system is a form of virtualisation that enables a host system to run many different operating systems. These hosted operating systems are referred to as guests and can be from different vendors and include Microsoft's Windows platforms and many GNU/Linux distributions.
Work is in progress to enable FreeBSD to run natively on xen using the 'enlightened' paravirtualisation method.
It is currently possible to run FreeBSD for i386 (32-bit) and amd64 (64-bit) using the hardware-assisted virtualisation instructions found on newer AMD and Intel processors. This ability came out with the XenServer 5.0 and Xen 3.2 releases (this was tested by the author using the official FreeBSD 7.0 current ISO releases).