
Dear Sana,
If you want show Purchase Order payment method only backend, you just need to declare
"protected $_canUseCheckout = false;" in the
file: app\code\core\Mage\Payment\Model\Method\Purchaseorder.php so Purchase Order cannot work on frontend.
And vice versa you can declare "protected $_canUseInternal = false;" in class Mage_Payment_Model_Method_Purchaseorder so that Purchase Order cannot work in backend. If you still need help regarding Magento Development feel free to contact me.
Hope you success.