Q:

Understanding JVM Better

I want to know the internals of the JVM, how it performs and the details that matters. Can you suggest any resource or best book available for this ??

A:

Chapters from "Inside the Java 2 Virtual Machine": 
http://www.artima.com/insidejvm/ed2/index.html
"Under the Hood" articles by Bill Venners
http://www.artima.com/underthehood/index.html
There's also "Jasmin", an assember for JVM, if you want to get your hands dirty and try low level programming with the JVM.
http://jasmin.sourceforge.net/

A:

Most of what I know of the JVM comes from the Sun docs. They have some great whitepapers on the HotSpot VM:
http://java.sun.com/javase/technologies/hotspot/index.jsp
You mentioned performance, so maybe you would be interested in the Sun performance docs:
http://java.sun.com/docs/performance/
For a more hands on approach, try looking at Kaffe. This will show you what a virtual machine does, but not necessarily how Java code works in Sun's JVM:

Kaffe is a clean room implementation of the Java virtual machine, plus the associated class libraries needed to provide a Java runtime environment.

And the OpenJDK is a great place to learn about how the Java Platform is implemented:
http://openjdk.java.net/

