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

Index: xen-4.0.0-testing/tools/python/xen/xend/image.py
===================================================================
--- xen-4.0.0-testing.orig/tools/python/xen/xend/image.py
+++ xen-4.0.0-testing/tools/python/xen/xend/image.py
@@ -1028,7 +1028,7 @@ 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))
         self.vramsize = int(vmConfig['platform'].get('videoram',4)) * 1024
 
     def buildDomain(self):