Taekwondo and boxing, while both being striking based martial arts, are completely different styles of fighting. One specializes in kicking techniques and the other specializes in punching.
In terms of effectiveness, both in MMA competition and in self defense applications, you've got to give the edge to boxing though both arts have plenty of holes in them and are best utilized within a well rounded arsenal.
In the following video from popular martial arts YouTube channel 'fightTIPS' we see a female taekwondo practitioner, Alex Wong who is a first degree blackbelt, spar with Shane who has some boxing experience under his belt.
For the first round it is solely punching vs kicking and it is an interesting clash of styles. Alex, the female taekwondo practitioner, is able to land her kicks from much farther away while Shane has to get real close to be able to land his strikes.
Alex has a very pure taekwondo style and tends to switch stances frequently depending on which kicking techniques she has up her sleeve as well as to disguise her attack and confuse her opponent.
A major issue with taekwondo, though, is that its practitioners tend to keep their hands down really low. This is because usually during TKD sparring and competition punches to the head are not allowed which tends to make these type of fighters more susceptible to head punches during mixed rules fights. Some high level fighters can pull it off though like UFC's Stephen 'Wonderboy' Thompson, who tends to keep his hands real low though he comes from a karate background, not TKD.
Another important factor is that the fighting area in this sparring session is pretty small which is a disadvantage for the TKD fighter who is going to need a lot of space to move around in to keep the fight at a long range where they are most effective.
All in all it is a pretty fun, light sparring session where you get to see clear examples of both styles in action.