All PVC/PSBM/PCS kernels (vzkernel-xxx RPMs) are based on upstream Red Hat kernels. The version of the Red Hat kernel is mentioned in the release notes of each Virtuozzo kernel. So you can check the corresponding KB article for any version of vzkernel.
Virtuozzo kernels with the same
XXXstabNNN suffix are based on the same Redhat kernel, change in the numbers means rebasing to the new kernel.
The current update for the Parallels Cloud Server 6.0 kernel provides a new kernel based on the Red Hat Enterprise Linux 6.3 kernel (2.6.32-279.22.1.el6).