JetBrains has a [wiki page][1] comparing Scala to Kotlin, and there seem to be a few things that Kotlin does and Scala does not: 

> - Zero-overhead null-safety. Scala has Option, which is a syntactic and run-time wrapper
> - [Smart casts][3]
> - Static extension functions. Instead of wrapping at runtime
> - Kotlin's Inline functions facilitate Nonlocal jumps
> - String templates. There's 3rd party compiler plugin for scala with similar functionality: ScalaEnhancedStrings
> - First-class delegation. Also implemented via 3rd party plugin: Autoproxy
Modules

So calling Kotlin a water down Scala is probably an oversimplification. As for Xtend, I think it targets mostly Xtext users, rather than a wider audience. A major difference to Scala is that Xtend compiles to Java rather than bytecode. 

Another "Java killer" language you should add to your list is [Red Hat's Ceylon][2], although I have no idea if and how it compares to Scala.

 [1]: http://confluence.jetbrains.net/display/Kotlin/Comparison+to+Scala
 [2]: http://en.wikipedia.org/wiki/Ceylon_Project
 [3]: http://kotlinlang.org/docs/reference/typecasts.html#smart-casts