There is no /proc/xen when booting kernel-ml-4.5.3-1.el7.elrepo.x86_64.
There is a /proc/xen when booting kernel-ml-4.5.0-1.el7.elrepo.x86_64 at least (as that is what I've fallen back too using). A simple reboot to kernel-ml-4.5.0 gets my new production system xen server running again.
The elrepo kernel 4.5.3 looks like it was built with xen support, however:
# grep -i xen /boot/config-4.5.3-1.el7.elrepo.x86_64
and is no different than the output from the last known working kernel of 4.5.0 (I have not yet been able to test 4.5.1 or 4.5.2)
They have asked me to file an upstream bug with you.
More details including the elrepo examination at:
Please hold off on looking into this issue for now. There may just be an issue with the grub.cfg not properly being updated to allow for xen to run. I should have confirmation Tuesday afternoon central USA time.
I can confirm that manually running /usr/bin/grub-bootxen.sh will properly allow for the /proc/xen to be populated. I will pursue this issue with the centos-virt peeps.
Resolving as this is not an issue with the kernel.