Can Product Owner and Scrum Master be the same person ?

No. Product Owner and Scrum Master are two separate roles and mixing them can have a very negative effect on the development process. Both role requires 100% involvement. One person will not be able to fulfil all his responsibilities in 100 %. Scrum Master sometimes needs to act as a mediator between the development team and PO when their goals start to diverge. In such case, if the same person is acting as both, there will be a conflict of interest.