HP doesn't offer 2720QM as an option on Envy. Only 2670QM or 2820QM. Its highly unlikely that they slipped in a processor which they don't give as an upgrade option normally.
Read this thread
for more understanding between the two processors.
Try using any benchmark software or opening any CPU intensive programs and see the maximum clock speed. If its 3.3 Ghz it might be the rare 2720QM on envy or if it gives 3.1 Ghz, it is the 2670QM.