かわいい approves through affection: the thing makes you want to protect it, keep it, smile at it. Smallness helps but is not required — what matters is the warmth of the reaction. It is subjective and personal in a way its English gloss is not, which is why Japanese speakers reach for it about handbags, cafés, phone cases, and each other, dozens of times a day.
きれい approves through admiration. A きれいな人 is beautiful — polished, composed, a little distant. A かわいい人 is endearing — you like them before you can say why. The two words split the territory English gives to “pretty”: きれい looks up at the thing, かわいい pulls it closer. Note the grammar splits too: きれいな needs な, かわいい never takes one.
かわいそう is not in this family. It is 可哀想, pitiful, and calling someone's appearance かわいそう tells them they look pathetic. The word you wanted for “kind of cute” is かわいらしい, or just かわいい with a hedge: ちょっとかわいい.
The katakana カワイイ names the aesthetic. When the word stops describing one puppy and starts describing a design language — mascots, pastel packaging, rounded letterforms — Japanese switches script, the same move that turns 微妙 into ビミョー. That is the カワイイ文化 the rest of the world imported wholesale.