That's unusual certainly; I'd expect it to pull it down if it needs it.
you could try
If that fails check the msg, but basically use something like
yum provides '*xen-libs*'
yum info '*xen-libs*'
I just did a quick google and found some solns.
Some people just did 'rpm --rebuilddb', some managed with 'yum clean all', but this is the most interesting/subtle one http://tech.kulish.com/2009/01/02/xe...issue-w-rhel5/