The xen_iret function in arch/x86/xen/xen-asm_32.S in the Linux kernel before 3.7.9 on 32-bit Xen paravirt_ops platforms does not properly handle an invalid value in the DS segment register, which allows guest OS users to gain guest OS privileges via a crafted application.
Xen 3.3.x through 4.5.x and the Linux kernel through 3.19.1 do not properly restrict access to PCI command registers, which might allow local guest OS users to cause a denial of service (non-maskable interrupt and host crash) by disabling the (1) memory or (2) I/O decoding for a PCI Express device and then accessing the device, which triggers an Unsupported Request (UR) response.
The make_response function in drivers/block/xen-blkback/blkback.c in the Linux kernel before 4.11.8 allows guest OS users to obtain sensitive information from host OS (or other guest OS) kernel memory by leveraging the copying of uninitialized padding fields in Xen block-interface response structures, aka XSA-216.