The do_mmu_update function in arch/x86/mm.c in Xen 4.x through 4.4.x does not properly restrict updates to only PV page tables, which allows remote PV guests to cause a denial of service (NULL pointer dereference) by leveraging hardware emulation services for HVM guests using Hardware Assisted Paging (HAP).
https://security.gentoo.org/glsa/201504-04
https://exchange.xforce.ibmcloud.com/vulnerabilities/98767
http://xenbits.xen.org/xsa/advisory-109.html
http://www.securityfocus.com/bid/71149
http://www.debian.org/security/2015/dsa-3140
http://secunia.com/advisories/62672
http://lists.opensuse.org/opensuse-security-announce/2015-02/msg00010.html
http://lists.opensuse.org/opensuse-security-announce/2015-02/msg00005.html