With Xen you can run multiple different kernels, even entirely different operating systems (as long as they support Xen).Depending on your requirements this can be a good thing or a bad thing. Do you actually need to run several different OSs or OS versions on your production servers? If not then virtualisation along the Zones/Jail line is actually a win since you don't have the resource hit (memory mainly, but also cycles) of running multiple kernel images at the same time.
