I see sloppy Administrators do this again and again…
They might update the Linux-Xen enabled Kernel on Dom0, but often DomU keeps the same for different reasons.
Running a (para) virtual environment, the freedom of running different Linux distributions, is often a goal. If one keeps a single architect environment stack, like Ubuntu Hardy Dom0 and DomU’s or CentOS 5.x Dom0 and DomU, keeping kernels in DomU up to date is low hassle.
The hassle starts to arise when you deploy mixed environments, like running Ubuntu Hardy as Dom0 and CentOS 5.x as DomU, or vice versa. You could setup CentOS or Ubuntu to use each others Kernel packages, though that seemed a bit overkill for my setup. Having a Debian Etch DomU on a Ubuntu Hardy Dom0 is fixable with pointing Etch to grab the Kernel from Hardy via an apt-repo.
But for the cases where I have a bit hassle, and I dont want to use PyGrub, I wrote a small bash script to update the Linux Kernels.
Get the script here, and update/change/modify or learn from it, before you use it.
It Powers down the DomU if it is booted, and mounts the Logical Volume of the DomU, before it copies the kernel modules to the DomU filesystem. Runs depmod and unmounts the filesystem. Then it gives you the small change you need to update your xen-domU.cfg with (I dont use pygrub).
BTW: This paper has a nice walk through from Xen DomU to Xen Dom0 bypassing SELinux http://invisiblethingslab.com/pub/xenfb-adventures-10.pdf. Recommended read
Now go and update some Kernels!