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

Index: xen-3.0.4-testing/tools/python/xen/xend/image.py
===================================================================
--- xen-3.0.4-testing.orig/tools/python/xen/xend/image.py
+++ xen-3.0.4-testing/tools/python/xen/xend/image.py
@@ -334,7 +334,7 @@ class HVMImageHandler(ImageHandler):
 
         self.dmargs += self.configVNC(imageConfig)
 
-        self.pae  = imageConfig['hvm'].get('pae', 0)
+        self.pae  = imageConfig['hvm'].get('pae', 1)
         self.apic  = imageConfig['hvm'].get('apic', 0)
         self.acpi  = imageConfig['hvm']['devices'].get('acpi', 0)