As far as I can see, XenServer itself still uses 2.6.18.
Running Xen Kernels I personally have used with it are 2.6.18 through 2.6.21 and now 2.6.31.
EDIT: I just noticed: for what would you need dom0 support? You only want to run a guest domain, a domU, for which there is stable support in the mainline kernel, iirc.
As far as I can see, XenServer itself still uses 2.6.18.
Running Xen Kernels I personally have used with it are 2.6.18 through 2.6.21 and now 2.6.31.
EDIT: I just noticed: for what would you need dom0 support? You only want to run a guest domain, a domU, for which there is stable support in the mainline kernel, iirc.