"v8 is not a browser engine either ,it's a javascript engine,just like Rhino. you are mixing up Webkit with v8."
- I think you're partly correct but also confused. Webkit is a rendering engine and yes V8 is a JavaScript engine not a browser engine that was created for Chrome Browser by Google. It's used for Node.JS etc now but it's sole purpose was to make Chrome faster and that is not the same purpose of Nashorn - nashorn is a replacement for Rhino. That is what I'm trying to say.
When the only thing you care about is performance that is a valid comparison. Besides, Nashorn is fast, was ment to be fast, and they are working on making it faster.