To install the package:
yum --enablerepo=virt-xen-VV-testing xen-vixen
Where VV is '44', '46', or '48', depending on which version you're using. (It's the same package for all versions.)
This will install the xen-vixen "shim" binary, as well as the pvshim-converter script.
See XSA-254 [1] for detailed information about who should use it, why, and when. Please report both successes and failures here. :-)
-George