What Qubes is is a Xen hypervisor (as I understand it, hypervisor independence is a work in progress) with a particular configuration and set of tools to serve as plumbing between various virtual machines and the dom0 host.
They distribute Fedora, Debian and Whonix domU guest images (and the dom0 host that is installed is Fedora), but you're free to install BSD, Windows, or whatever else you can get to run as a VM guest.
They distribute Fedora, Debian and Whonix domU guest images (and the dom0 host that is installed is Fedora), but you're free to install BSD, Windows, or whatever else you can get to run as a VM guest.