0006906: when use centos set up a vm, the vm's io is poor

I hava set up a vm on the host which installed centos and rhel6.3,the same raid and same cpu,then i found the vm's disk io are different,the vm on centos which disk io is poor. is there any change on the driver for rhel6.3 and centos 6.4,the machine is dell r620.