I have a form that looks like this:
You won't be able to create or edit a local XML file using javascript/jquery because of security concerns. Imagine going to a website and the webmaster wrote some code doing who knows what and puts this in a file on your computer...
The only way to write to a local file using javascript is to use cookies or HTML5 localStorage.
localStorage will allow you to store string keys and values of arrays and/or property names and values of objects.
If you need an XML file you will have to call a server-side script that has permission to write the file on the server, which you can then access via it's url.