Its not really clear.
I read the Hold the Line feat as saying that a PC with that Feat treats entering a threatened area as an AoO-provoking event that is distinct from leaving a threatened area.
Thus, if a being enters an area threatened by a PC with Hold the Line, he provokes an AoO. If he then continues his movement, he provokes an AoO for leaving a threatened area. However, a creature moving through your threatened areas can only provoke 1 AoO for entering and 1 AoO for leaving a threatened area, no matter how many times he does so in that turn.
I say this because the wording of AoOs doesn't mention 1 AoO "per movement"- just 1 AoO for leaving a threatened area no matter how many times it is done that turn. Under the basic, unmodified rules, there is only one type of movement that provokes an AoO (leaving a threatened area)- Hold the Line adds another movement type (entering a threatened area).