Xen 4.1.1 and earlier allows local guest OS kernels with control of a PCI[E] device to cause a denial of service (CPU consumption and host hang) via many crafted DMA requests that are denied by the IOMMU, which triggers a livelock.
http://www.securityfocus.com/bid/49146
http://www.debian.org/security/2012/dsa-2582
http://secunia.com/advisories/51468
http://secunia.com/advisories/45622
http://old-list-archives.xen.org/archives/html/xen-devel/2011-08/msg00450.html
http://old-list-archives.xen.org/archives/html/xen-devel/2011-06/msg01106.html