It depends on what you used to create the PDF Form.
If you used LiveCycle Designer, then it is quite easy. You can bind your XML strucuture to the PDF fields using either a schema, or a sample XML file. Then LiveCycle Forms ES (a server side module running in the LiveCycle environment) can extract the data directly in your XML format.
If the form is an "Acro Form" then you can extract the data using either the LiveCycle Forms ES, or LiveCycle Form Data Integration module. You'll then have to programatically transform it into the XML format.
If its something else, such as a non-interactive flat file, then things get more complicated
I would prefer to use LC Designer, can you explain little bit more on how to start it and LC Form ES.
I've noticed that you are already very active on the Designer forums. There is additional information in the documentation: http://help.adobe.com/en_US/livecycle/9.0/designerHelp/index.htm
You can find more about Live Cycle Forms and the other Live Cycle Modules in the Developer Center: http://www.adobe.com/devnet/livecycle.html