Question

difference between well formed document and validated document

Answer 1

A well-formed XML document complies with the syntactic rules of XML markup. These rules are strict but basically quite simple. The XML syntax rules include

documents must be self-describing (they start with an XML declaration)a document must contain one or more element documents must contain a single root element start and end tags must be used to identify elements (must have closing tags)empty elements must be marked as such (with a self-closing />)attribute values must be quoted (single or double quotes are fine)element names and attributes names are case sensitive.element tags must be correctly nested (must not overlap)element names (attribute names) must start with a letter.
A valid XML document means that the document complies with the rules of a particular document specification. The specification is commonly done in an external DTD file, however, it might also be a DTD section of the XML file, or an XML based schema file (which also allows other features)