The remote host is missing an update to xen announced via advisory DSA 2337-1.

https://secure1.securityspace.com/smysecure/catid.html?in=DSA%202337-1

Several vulnerabilities were discovered in the Xen virtual machine hypervisor. CVE-2011-1166 A 64-bit guest can get one of its vCPU'ss into non-kernel mode without first providing a valid non-kernel pagetable, thereby locking up the host system. CVE-2011-1583, CVE-2011-3262 Local users can cause a denial of service and possibly execute arbitrary code via a crafted paravirtualised guest kernel image. CVE-2011-1898 When using PCI passthrough on Intel VT-d chipsets that do not have interrupt remapping, guest OS can users to gain host OS privileges by writing to the interrupt injection registers. The oldstable distribution (lenny) contains a different version of Xen not affected by these problems. For the stable distribution (squeeze), this problem has been fixed in version 4.0.1-4. For the testing (wheezy) and unstable distribution (sid), this problem has been fixed in version 4.1.1-1. We recommend that you upgrade your xen packages.