Iaido and iaijutsu are essentially the same thing: the art of drawing the sword and cutting in a single motion. Iaido and iaijutsu utilize kata as the primary mechanism of training. There is no sparring. There is a deep philosophical element to ZNKR iaido, with the object of improving the practitioner as a person. Having said that, a skilled iaido practitioner with a shinken could easily kill someone with the skills they have learned.
Kendo is fencing, and looks more like sword fighting than iaido. Kendo has a kata element (paired kata) where bokuto or actual (though un-edged) swords are utilized. One could certainly, with a shinken, kill a person with the skill that they have learned in kendo.
Kendo revolves around shiai; the match or bout. Bouts in kendo are full contact. Iaido revolves around kata practiced individually; I believe that there are thirteen ZNKR iaido kata. There is no sparring in ZNKR iaido. Nobody cares. They don't practice iaido for the purpose of winning matches.
Actually, taekwondo is more like kendo. It is a modern unarmed art that is trained through kata (pumse/tul/hyeong/whatever Korean term is used) and shihap (sparring contests). The purpose of the sparring is to score points, be it in continuous play or in point/stop sparring. Belts, gender, and age are used for competition bracketing.
The sparring in taekwondo does not resemble any real fight that you will see, regardless of which federation you are dealing with and regardless of the contact level or amount of protective gear worn.
The purpose of taekwondo is similar to the purpose of kendo: to improve the practitioner as a person and member of collective humanity through the practice of taekwondo. Yes, you can defend yourself with taekwondo. But it is not the primary purpose of the art, and not everyone takes it specifically for self defense.