Question

What is the difference between a valid XML document and a "well formed" XML document?

Answer 1

1) Well Formed XML documents are the one that meets all the well-formedness constraints given in this specification.

XML document are well formed if they are syntactically correct

i.e. An XML document with correct syntax is called "Well Formed".

2) Valid XML A Well Formed XML is a valid XML, when the well-formed XML document follows the rules (document structure and rule) specified by the XML Schema or DTD.