There are 2 main differences between Shelly Plus 1PM and Shelly Plus 1:
1) Dry contacts - Shelly Plus 1 has dry contacts, while Shelly Plus 1PM does not. This means that with Shelly Plus 1 you can connect one type of voltage for powering the device (12V, 24-60V DC or 110V-230V AC) and another type of voltage for powering the load. This is not possible for Shelly Plus 1PM.
2) Power measurement - Shelly Plus 1PM has power measurements and temperature and overload protection.