Hello Arun,
Since your question is not straight forward it is difficult to answer to your query because you speak about PO/PR approval as well as email notification for the same in one question.
1. For PO/PR approval email notification
you have standard email notification
Home ----> inbox --> subscribe to tasks
or
create a custom email or BTM Task by following below link
Note : We dont have approval process for purchase request but only project purchase request is possible
Regards
Leo