Boom is just a "boom" while KAboom refers to the "WOW" effect verison of boom. Something truly boom, becomes kaboom.
Can sometimes be written with multiple "o"'s such as: kabooooooom.
Meaning an even larger boom than kaboom.
Usually said with a special voice or sounds. Putting weight on the "o"s.
ps: if you actually say this, you might be considered a retard as it sounds pretty stupid saying this.
Also an onomatopoeia of a huge explosion
Guy 1: Wow, that was one large explosion!
Guy 2: WHAAAT?!?!?!
2. Guy 1: KABOOOM. have a look at that chick.
"I dunno, he totally kabooms everything, ya kno?"
"Sheewt, I wish I could kaboom like Billy Mays can."