Question: Can someone explain the differences between GraalVM and Jvm-Hotspot, and how GraalVM is related to OpenJDK's HotSpot.

Answer: 
In that context you could think of GraalVM as a modified/improved HotSpot JVM [very loosely: OpenJDK's HotSpot + the Graal JIT Compiler (replacing the one from HotSpot) + the ability to run other languages (JS, Ruby, R, Python, etc.) + ability to AOT compile Java 
 + additional bundled tools, etc.]
GraalVM is a JVM with multiple change:

* the graal compiler. It is intended to replace the C2 compiler (server mode) in front of HotSpot.
* Truffle a library that you can implements to make JVM polyglot ( there already a number of language that are supported like js, R, Ruby...)

The graal project also have a Substrate VM which is a framework to build native artifact from Java.