The difference between sino-atrial node (SAN) and atrio-ventricular node (AVN) is:

Both SAN and AVN the ability to generate action potentials without any external stimuli (autoexcitable). The SAN can generate the maximum number of action potentials, i.e., 70-75 min–1, and is responsible for initiating and maintaining the rhythmic contractile activity of the heart. Therefore, it is called the pacemaker.SAN is present in the upper right corner of the right atrium whereas AVN is present in the lower left corner of the right atrium. SAN acts as the pacemaker whereas AVN acts as a relay station.