HVM guests are currently permitted to modify the memory and I/O decode bits in the PCI command register of devices passed through to them. Unless the device is an SR-IOV virtual function, after disabling one or both of these bits subsequent accesses to the MMIO or I/O port ranges would - on PCI Express devices - lead to Unsupported Request responses. The treatment of such errors is platform specific. Furthermore (at least) devices under control of the Linux pciback driver in the host are handed to guests with the aforementioned bits turned off. This means that such accesses can similarly lead to Unsupported Request responses until these flags are set as needed by the guest.
In the event that the platform surfaces aforementioned UR responses as Non-Maskable Interrupts, and either the OS is configured to treat NMIs as fatal or (e.g. via ACPI's APEI) the platform tells the OS to treat these errors as fatal, the host would crash, leading to a Denial of Service.
Xen versions 3.3 and onwards are vulnerable due to supporting PCI pass-through. Only x86 systems are vulnerable. ARM systems are not vulnerable. Only HVM guests with their device model run in Dom0 can take advantage of this vulnerability. Any domain which is given access to a non-SR-IOV virtual function PCI Express device can take advantage of this vulnerability.