Dementors use Dementor’s Kiss to suck out the soul. A human can survive without a soul in something like a vegetative state.
“They call it the Dementor’s Kiss,” said Lupin, with a slightly twisted smile. “It’s what Dementors do to those they wish to destroy utterly. I suppose there must be some kind of mouth under there, because they clamp their jaws upon the mouth of the victim and — and suck out his soul.”
Harry accidentally spat out a bit of butterbeer. “What — they kill —?”
“Oh no,” said Lupin. “Much worse than that. You can exist without your soul, you know, as long as your brain and heart are still working. But you’ll have no sense of self anymore, no memory, no... anything. There’s no chance at all of recovery. You’ll just exist. As an empty shell. And your soul is gone forever... lost.”
Harry Potter and the Prisoner of Azkaban, Chapter 12 , The Patronus
While the Killing Curse kills the person without causing any harm to the body.
“Avada Kedavra!” Moody roared.
There was a flash of blinding green light and a rushing sound, as though a vast, invisible something was soaring through the air - instantaneously the spider rolled over onto its back, unmarked, but unmistakably dead. Several of the students stifled cries; Ron had thrown himself backward and almost toppled off his seat as the spider skidded toward him.
Moody swept the dead spider off the desk onto the floor.
“Not nice,” he said calmly. “Not pleasant. And there’s no countercurse. There’s no blocking it. Only one known person has ever survived it, and he’s sitting right in front of me.”
Harry felt his face redden as Moody’s eyes (both of them) looked into his own. He could feel everyone else looking around at him too. Harry stared at the blank blackboard as though fascinated by it, but not really seeing it at all...
Harry Potter and the Goblet of Fire, Chapter 14 , The Unforgivable Curses
The cases quoted by you are special cases.
1. When the Killing Curse rebounds on Lord Voldemort the first time, a
fragment of his soul blows into Harry's.
JKR: So because Voldemort never went through the grotesque process that I imagine creates a Horcrux with Harry, (SU: Mm-hm.) it was just that he had destabilized his soul so much that it split when he was hit by the backfiring curse. And so this part of it flies off, and attaches to the only living thing in the room. A part of it flees in the very-close-to-death limbo state that Voldemort then goes on and exists in.
The Leaky Cauldron, Transcript of part 1 of Pottercast’s JK Rowling interview
2. When Voldemort kills Harry, that same fragment gets blown off.
I can't answer this better than Why didn't Harry die in the dark forest?
3. When Voldemort kills his family, their bodies are in perfect working
order, except they're dead.
That's how the killing curse works.
“The Muggle authorities were perplexed. As far as I am aware, they do not know to this day how the Riddles died, for the Avada Kedavra curse does not usually leave any sign of damage... The exception sits before me,” Dumbledore added, with a nod to Harry’s scar. “The Ministry, on the other hand, knew at once that this was a wizard’s murder. They also knew that a convicted Muggle-hater lived across the valley from the Riddle house, a Muggle-hater who had already been imprisoned once for attacking one of the murdered people.
Harry Potter and the Half-Blood Prince, Chapter 17 , A Sluggish Memory