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

Index: xen-3.0.3-testing/tools/python/xen/xend/image.py
===================================================================
--- xen-3.0.3-testing.orig/tools/python/xen/xend/image.py
+++ xen-3.0.3-testing/tools/python/xen/xend/image.py
@@ -263,7 +263,7 @@ class HVMImageHandler(ImageHandler):
 
         self.dmargs += self.configVNC(imageConfig)
 
-        self.pae  = int(sxp.child_value(imageConfig, 'pae', 0))
+        self.pae  = int(sxp.child_value(imageConfig, 'pae', 1))
 
         self.acpi = int(sxp.child_value(imageConfig, 'acpi', 0))
         self.apic = int(sxp.child_value(imageConfig, 'apic', 0))