I first thought that Xen was a host hypervisor. But i later found that it was a bare metal hypervisor. How does Xen qualify as a Bare metal hypervisor ?
Because Xen can be booted to directly from BIOS, without any other operating system in between Xen and the hardware. VMware ESXi is similar (differentiating itself from VMware workstation which requires a host OS).
Xen's dom0 privleged guest can be installed on the system before Xen is enabled, thus confusing matters about whether it is a bare metal install or not, but it is technically a true bare metal install that starts the dom0 guest. Note that you can often boot an alternate kernel which will bypass Xen and boot the dom0 "guest" as a true non-virtualized host (without Xen enabled).
Another bare metal hypervisors are also RedHat Vitalization Hypervisor, and opensource oVirt.
来源：https://stackoverflow.com/questions/6100949/xen-bare-metal-hypervisor