When you attempt to create a guest, you receive an "Invalid argument" error message. This usually means that the kernel image you are trying to boot is incompatible with the hypervisor. An example of this would be if you were attempting to run a non-PAE FC5 kernel on a PAE only FC6 hypervisor.

You do a yum update and receive a new kernel, the grub.conf default kernel switches right back to a bare-metal kernel instead of the Virtualization kernel.

To correct this problem you must modify the default kernel RPM that resides in the /etc/sysconfig/kernel/ directory. You must ensure that kernel-xen parameter is set as the default option in your gb.conf file.