1. I don't know the exact difference. But I know that XenSource the company behind Xen was bought by Citrix. So I guess they have a different product. Since Xen is Open Source it's been included with several distros in cluding RHEL, CentOS, Fedora and Ubuntu.
2. Why xm command doesn't work: Check whether you have the necessary packages. For example just having a Xen kernel (kernel-xen) doesn't guaruntee the availability if xen utilities.
Find what's installed
Search the repos for what are available.
If xen, xen-libs or any other necessary package is not installed try installing them with
#yum install xen xen-libs
You might some from here
3. Try virt-manager. It's a GUI tool which can help you to manage Xen VMs. Also if your time allows ans need drives, have a look at the new oVirt