What's the difference between positive and negative pressure relief valves?
The positive pressure relief valve opens in case the cabin pressure is too high in comparison to outside air Pressure. This happens if the pressure differential is higher than 8.85 psi / 0.61 bar (Airbus). It is most likely to happen in malfunction of the outflow valve (stays closed).

The negative pressure relief valve opens most likely during descent, when the cabin altitude is still at 8,000 feet (normal condition) but the aircraft itself is lower than that. That means that the pressure outside of the aircraft is higher than inside. The valve opens at a differential pressure of -1 psi / 0.07 bar.