I've used Xen, VMware, and VirtualBox. I have had the best results with VirtualBox. It may have changed, but it used to be that you could not install a native nVidia video driver on a system running Xen, so you could not get full use out of modern displays. That may have been fixed (it was 5 years ago when I last used it). That's why I changed to VirtualBox. I've been using it ever since on both Linux and Windows hosts, running a variety of operating systems including Linux, QNX, DOS, Windows, Solaris, and BSD Unix. Works great, and it is FOSS (Free and Open Source Software), so you can even get the source code for it.