Here's what the FAQ says:
> If you really want to call it a distribution, then it’s more of a “Xen distribution” than a Linux one. But Qubes is much more than just Xen packaging. It has its own VM management infrastructure, with support for template VMs, centralized VM updating, etc. It also has a very unique GUI virtualization infrastructure.
Xen is not like VirtualBox (which runs on a kernel, it's called a type II hypervisor), Xen is a bare metal type 1 hypervisor, again from the link I mentioned earlier by Qubes founder:
> First, products such as VMWare Workstation or Fusion, or Virtual Box, are all examples of type II hypervisors (sometimes called “hosted VMMs”), which means that they run inside a normal OS, such as Windows, as ordinary processes and/or kernel modules. This means that they use the OS-provided services for all sorts of things, from networking, USB stacks, to graphics output and keyboard and mouse input, which in turn implies they can be only as secure as the hosting OS is. If the hosting OS got compromised, perhaps via a bug in its DHCP client, or USB driver, then it is a game over, also for all your VMs.
Xen is absolutely not like those, but I do understand from where your misunderstanding came from (maybe treating Xen as a similar VirtualBox?).
It's based on Xen (but it uses Linux (fedora) in dom0).