What are fixed rate pacemaker and demand type pacemaker?

Same: Pacemakers have both capabiloties. Most pacemakers are programmed demand pacemaker. It will ensure the fixed lowest heart rate and it can increase heart rate depending on your activities need. Sometimes rate response ( rate increasing capabilities) can be turned off if needed. Everyone does not need rate response.

Pacemakers: Fixed rate pacemakers stimulate the heart to beat all the time at a specified rate. Demand pacemakers only pace the heart if the rate falls below a certain set rate or irregularity in heartbeat occurs.