I decided after a few problems that I would not upgrade my Debian machines from wheezy to jessie. jessie appears to be more oriented towards workstations, and the decision to set systemd as the only realistic boot loader in jessie caused concerns.
One of our technicians upgraded a Debian Wheezy Xen DOM0 in early fall of 2017, and while it rebooted, it could no longer run hardware virtualized machines (hvm's). I was called in to solve the issue and failed.
Finally, after deciding the only solution was a complete reinstall, I decided to test out the devuan distribution (http://devuan.org/), which was created to develop a systemd free alternative to Debian. Within an hour, the system was up and running.
Following are my notes on how this was accomplished. Most were taken from https://git.devuan.org/dev1fanboy/Upgrade-Install-Devuan/wikis/Upgrade-to-Devuan
Note that the step, install qemu-system-x86 is possibly the only fix that was necessary. I could still be running Debian Wheezy, probably. However, by upgrading to devuan, I get new kernel's and packages, and it was only after the upgrade I got useful error messages leading me to install this.
Basically, do as you normally do. The system will be broken after this
apt-get update && apt-get upgrade
Edit /etc/apt/sources.list and remove all entries pointing to debian, then add the following lines (I remove them by commenting them out)
deb http://auto.mirror.devuan.org/merged jessie main deb http://auto.mirror.devuan.org/merged jessie-updates main deb http://auto.mirror.devuan.org/merged jessie-security main
In most cases, these should be the only lines in sources.list
Now, import the devuan keyring
apt-get update && apt-get install devuan-keyring -y --allow-unauthenticated
apt-get update -y apt-get dist-upgrade
The -y is needed or it will complain about the keyring the first time.
reboot the system
Appearently, someone has broken the qemu system into pieces in this upgrade. Good idea; if you are only going to use paravirtualized domu's, there is no need for it. But, if you are using hardware virtualized, this is what makes it work.
apt-get install qemu-system-x86
The configuration options for hvm's has changed, with many not being used. In /etc/xen, there is a sample for hvm's which will help a lot, however the following is a quick and dirty list.
Again, read the sample conf, but basically, remove anything not needed.