Hi Karthi,
1. You can attach a schema to a form in Form Builder, allowing you to use the single types in the schema to validate fields. But this is not to validate the XML produced by Orbeon Forms.
http://doc.orbeon.com/form-builder/advanced/xml-schema-support.html2. There is no built-in facility to map the XML format produced by Form Builder to another XML format, and we'd indeed recommend you use XSTL for this.
I hope this helps,
Alex
--
Follow Orbeon on Twitter: @orbeon
Follow me on Twitter: @avernet