If the Troubleshooting section above does not help you to solve your problem, check the Red Hat Bugzilla for existing bug reports on Xen in FC6. The product is "Fedora Core", and the component is "kernel" for bugs related to the xen kernel and "xen" for bugs related to the tools. These reports contain useful advice from fellow xen testers and often describe work-arounds.
For general Xen issues and useful information check the Xen project documentation, and mailing list archives.
Finally, discussion on Fedora Xen support issues occur on the Fedora Xen mailing list