Dom0, or domain zero to expand the abbreviation, is the first domain started by the Xen hypervisor on boot. It has special privileges, like being able to cause new domains to start, and being able to access the hardware directly. Unless DriverDomains are being used, it is responsible for running all of the device drivers for the hardware. For hardware that is made available to other domains, like network interfaces and disks, it will run the BackendDriver, which multiplexes and forwards to the hardware requests from the FrontendDriver in each DomU.
Modified versions of Linux, NetBSD and Solaris can be used as the dom0. See OS vendor documentation for specific steps. Current official documentation may be found at the following sites:
NetBSD -- http://www.netbsd.org/ports/xen/howto.html#netbsd-dom0
OpenSolaris -- http://www.opensolaris.org/os/community/xen/docs/SetupDom0.htm
For info about available Linux Xen dom0 kernels please see this wiki page: http://wiki.xensource.com/xenwiki/XenDom0Kernels
