Document nova in tree virt drivers

In I29c5a678efec4fbc4bd7958ebe6d454ae30701cd we made the libvirt driver
to run only on Linux. However we did not have documentation about what
virt driver runs on what Operating System. This patch starts such
documentation.

Change-Id: Iccf4ab14865ac1694d7b0dad5dcb101f1ba152c8

doc/source/admin/configuration/hypervisors.rst

@@ -71,6 +71,34 @@ The following hypervisors are supported:
 * `Ironic`_ - OpenStack project which provisions bare metal (as opposed to virtual)
   machines.
 
+
+Nova supports hypervisors via virt drivers. Nova has the following in tree
+virt drivers:
+
+* :oslo.config:option:`compute_driver` = ``libvirt.LibvirtDriver``
+
+  This driver runs on Linux and supports multiple hypervisor backends, which
+  can be configured via the :oslo.config:option:`libvirt.virt_type` config
+  option.
+
+* :oslo.config:option:`compute_driver` = ``xenapi.XenAPIDriver``
+
+* :oslo.config:option:`compute_driver` = ``ironic.IronicDriver``
+
+* :oslo.config:option:`compute_driver` = ``vmwareapi.VMwareVCDriver``
+
+* :oslo.config:option:`compute_driver` = ``hyperv.HyperVDriver``
+
+* :oslo.config:option:`compute_driver` = ``powervm.PowerVMDriver``
+
+* :oslo.config:option:`compute_driver` = ``zvm.ZVMDriver``
+
+* :oslo.config:option:`compute_driver` = ``fake.FakeDriver``
+
+  This driver does not spawn any virtual machines and therefore should only be
+  used during testing.
+
+
 .. _KVM: https://www.linux-kvm.org/page/Main_Page
 .. _LXC: https://linuxcontainers.org
 .. _QEMU: https://wiki.qemu.org/Manual