What's the actual difference between edge sensitive and level sensitive interrupts
Its exactlly what is says.

If edge interrupt is set, the ISR will only get fired on falling/rising edge of a pulse. While if level sensitive interrupt (as you say) is set the ISR will get fired everytime there is a low-level/high-level signal on the corresponding pin.

In short, edge interrupt gets fired only on changing edges, while level interrupts gets fired as long as the pulse is low or high.

So if you have low-level interrupt set, MCU will keep executing the ISR as long as the pin is low.