[CentOS-virt] Is kvm-clock dependant on hardware of Physical Server?

I've got a three Centos 5.5 KVM severs, but only one of them is presenting guests with the kvm-clock source.

The two servers I'm not seeing kvm-clock on have Athlon x2 64bit processors.

The other server with kvm-clock has a Phenom II X6 1090T Processor.

I've checked the system configuration and all the boxes have the same set of RPM packages. To confirm that the issue was physical hardware specific I migrated a VM from the Athlon to the Phenom server and the kvm-clock source appeared.

Any ideas why I'm not seeing kvm-clock with my Athlon X2 processors, or is there another difference I haven't caught?

Steve