Xen 4.6.x, 4.5.x, 4.4.x, 4.3.x, and earlier do not initialize x86 FPU stack and XMM registers when XSAVE/XRSTOR are not used to manage guest extended register state, which allows local guest domains to obtain sensitive information from other domains via unspecified vectors.
https://security.gentoo.org/glsa/201604-03
http://xenbits.xen.org/xsa/advisory-165.html
http://www.securitytracker.com/id/1034477
http://www.securityfocus.com/bid/79543
http://www.oracle.com/technetwork/topics/security/ovmbulletinjul2016-3090546.html