Xen offers the ability to run PV backends in regular unprivileged guests, typically referred to as "driver domains". Running PV backends in driver domains has one primary security advantage: if a driver domain gets compromised, it doesn't have the privileges to take over the system.

However, a malicious driver domain could try to attack other guests via sending events at a high frequency leading to a Denial of Service in the guest due to trying to service interrupts for elongated amounts of time.

There are three affected backends:
 * blkfront          patch 1, CVE-2021-28711
 * netfront          patch 2, CVE-2021-28712
 * hvc_xen (console) patch 3, CVE-2021-28713

Reference:
https://xenbits.xen.org/xsa/advisory-391.html