PAE must be on for 64-on-64 to work at all.

Index: xen-3.3.1-testing/tools/python/xen/xend/image.py
===================================================================
--- xen-3.3.1-testing.orig/tools/python/xen/xend/image.py
+++ xen-3.3.1-testing/tools/python/xen/xend/image.py
@@ -871,7 +871,7 @@ class X86_HVM_ImageHandler(HVMImageHandl
 
     def configure(self, vmConfig):
         HVMImageHandler.configure(self, vmConfig)
-        self.pae = int(vmConfig['platform'].get('pae',  0))
+        self.pae = int(vmConfig['platform'].get('pae',  1))
 
     def buildDomain(self):
         xc.hvm_set_param(self.vm.getDomid(), HVM_PARAM_PAE_ENABLED, self.pae)