I'm guessing Xen PV isn't well tested by Red Hat anymore since most (if not all) of their paying customers still (stuck) using it are likely on RHEL5, which they haven't released a patch for yet due to that very reason.

I'm kind of shocked Amazon doesn't have something like Linode's Fennix, but you can always do an EBS snapshot of your /boot volume and revert it if a kernel upgrade breaks stuff.