The compiler can emit optimizations in the PV backend drivers which can lead to double fetch vulnerabilities. Specifically the shared memory between the frontend and backend can be fetched twice (during which time the frontend can alter the contents) possibly leading to arbitrary code execution in backend.
Malicious guest administrators can cause denial of service. If driver domains are not in use, the impact can be a host crash, or privilege escalation.
Systems running PV or HVM guests are vulnerable. ARM and x86 systems are vulnerable. All OSes providing PV backends are susceptible, this includes Linux and NetBSD. By default the Linux distributions compile kernels with optimizations.