There has been a fair amount of press in the last couple of days concerning Xen update XSA-108, and the fact that Amazon EC2 and Rackspace must reboot after this update:
Rackspace forced to reboot cloud over Xen bug
Amazon Reboots Cloud Servers, Xen Bug Blamed
There are other stories, but those articles cover the main issue.
As KB tweeted, the CentOS-6 Xen4CentOS release is also impacted by this issue and the CentOS team has released CESA-2014:X013 to deal with XSA-108. There are also 3 other Xen4CentOS updates released: CESA-2014:X010, CESA-2014:X011, CESA-2014:X012
If you are using Xen4CentOS on CentOS-6, please use yum update to get these security updates ... and like Rackspace and Amazon EC2, you need to reboot your dom0 machine after the updates are applied.