I have CentOS 6.4 i386/686 installed on its own.I'd like to test out Samba 4 running as an AD DC.  If I don't have another spare box to use in the lab, is it possible / does it make sense to install XP Pro on a VM within CentOS so I can test whether XP Pro clients will be able to make use of Samba's services OK?  If so, is there anything special to consider with this setup, or would I just install KVM and follow something like this guide?  (Or Xen?)
Samba test setup with only one physical box
centos;windows;samba;kvm;active directory
This is a perfectly valid approach to doing the setup and testing. I often do the same thing for development and testing of client/server applications all the time. The only thing you'll need to take care when setting up the VM is to make sure that it's using a network device that has been bridged to the host's network device so that it (the VM) can be a full participant on the network. When setting up VMs, setting up the network is often the biggest stumbling block. Here's a excellent tutorial that covers all the types of networking, titled: VMWare Interfaces Tutorial. The tutorial is geared to VMWare and it's primary done on windows but the diagrams are excellent at showing the differences between the types of configurations you can apply when setting up networking for a VM.