Expand platform docs with Xen PV example

Signed-off-by: Justin Cormack <justin.cormack@docker.com>

docs/platforms.md

@@ -3,21 +3,38 @@
 These are the supported platforms. Each should boot with `mobyplatform=xxx` in the command line.
 
 ### Desktop
-`mac`
-`windows`
+`mac` `windows` https://github.com/docker/pinata
 
-https://github.com/docker/pinata
+The desktop editions.
 
 ### Cloud
-`aws`
-`azure`
+`aws` `azure` https://github.com/docker/editions
 
-https://github.com/docker/editions
+The cloud editions
 
 ### Test
-`test`
+`test` https://github.com/docker/moby
 
-https://github.com/docker/moby
+Internal test target
 
-### Other
-KVM/machine https://github.com/docker/moby/pull/225
+### Running on other platforms
+`unknown`
+
+The default fallback target name is `unknown` that does only default setup.
+
+## Notes on other platforms
+
+KVM/machine: https://github.com/docker/moby/pull/225
+
+Xen PV: a suitable config is
+```
+name = "moby0"
+memory = 1024
+vcpus=1
+disk = ['phy:/dev/vg0/moby0,hda,w']
+vif = [ 'mac=00:22:ab:42:99:00, bridge=br0' ]
+kernel="/home/justin/images/moby/vmlinuz64"
+ramdisk="/home/justin/images/moby/initrd.img.gz"
+extra="console=hvc0"
+on_reboot="restart"
+```