In order to test the templates on Xenserver we had to run this HV as a KVM guest (gotta love virtualisation!); however by default Xenserver will complain that you can't run any HVM guests, only paravirt ones (PV). This sucks because PV is used less and less with HVM being in the spotlight.
Luckily with KVM we can forward the VMX CPU flag to a guest and as such make it available to Xenserver, for it's HVM mode.
There are a few things to be aware of though:
1 - in libvirt do give the Xenserver VM a good CPU profile (I used Core2duo) and make sure the VMX flag is set on "require"
2 - stock CentOS 7 kernel has a problem with nested virt at the moment, do use a newer kernel (I'm using kernel-ml from elrepo-kernel)
3 - make sure the kvm_intel module is loaded with the option nested=1. For this to happen I reload/rebooted with this in /etc/modprobe.d/kvm-intel.conf:
options kvm-intel nested=1
Now enjoy docker on centos, in xenserver on kvm on centos. :-)
 - https://bugzilla.kernel.org/show_bug.cgi?id=45931 - this will likely be fixed in future CentOS/RH kernel updates, I hope