Xen 4.1.x and 4.2.x, when the XSA-45 patch is in place, does not properly maintain references on pages stored for deferred cleanup, which allows local PV guest kernels to cause a denial of service (premature page free and hypervisor crash) or possibly gain privileges via unspecified vectors.
Debian Linux Security Advisory 3006-1 - Multiple security issues have been discovered in the Xen virtualisation solution which may result in information leaks or denial of service.