Kotlin: Void, Unit and Nothing — what's the difference and when to use what?

Hello everyone! I recently started learning Kotlin and switched from Java. In Java, I'm used to void for methods that return nothing. But in Kotlin, I see three similar concepts: Void, Unit, and Nothing. And I'm confused.

For example, I write a function that simply prints a greeting:

fun greet(name: String) {
    println("Hello, $name!")
}

The compiler says the return type is Unit. But I can also write it like this:

fun greet(name: String): Unit {
    println("Hello, $name!")
}

Also, I've seen Void (capitalized) and Nothing in someone else's code. When I tried to use Nothing as a return type, the compiler said the function never completes normally.

Please explain:

1. How are Unit, Void, and Nothing different?
2. When should I use