In the given figure, V₀ is the potential barrier across as p-n junction, when no battery is connected across the junction (2 is corresponding to unbiased diode).

1 and 3 both correspond to forward bias of junction

3 corresponds to forward bias and 1 corresponds to reverse bias of junction

1 corresponds to forward bias and 3 corresponds to reverse bias of junction

3 and 1 both correspond to reverse bias of junction