Supervisor Mode Access Prevention is a hardware feature designed to make an Operating System more robust, by raising a pagefault rather than accidentally following a pointer into userspace. However, legitimate accesses into userspace require whitelisting, and the exception delivery mechanism for 32bit PV guests wasn't whitelisted.
A malicious 32-bit PV guest kernel can trigger a safety check, crashing the hypervisor and causing a denial of service to other VMs on the host.
Xen version 4.5 and newer are vulnerable. Versions 4.4 and older are not, due to not having software support for SMAP. The vulnerability is only exposed on x86 hardware supporting the SMAP feature (Intel Broadwell and later CPUs). The vulnerability is not exposed on ARM hardware, or x86 hardware which do not support SMAP. The vulnerability is only exposed to x86 32bit PV guests. The vulnerability is not exposed to 64bit PV guests or HVM guests.