vmotion Error

Hi all

I have VMware ESXi 5.1 four server

2 Hosts  (Dell PE R 710) are with following CPU configuration :-

Intel(R) Xeon(R) CPU X5650 @ 2.67GHz (6 Core) = 12 CPUs x 2.66 Ghz Logical Processors: 24

2 Hosts (Dell PE R720) are with following CPU configuration :-

Intel(R) Xeon(R) CPU E5-2665 0 @ 2.40GHz (8 Core) = 16 CPUs x 2.40 Ghz Logical Processors : 32

But while vmotion we are getting error,  we have tried al VMware EVC option but still we are getting error. pls advice