A valid XML document is the one which conforms to all the 
rules stated in its associated DTD or a schema as well as 
it is well formed.

A well formed XML document fullfills the following 
conditions:
1. All opened tags are closed.
2. Tags are properly nested
3. Attribute values are enclosed in double quotes.
4. Tags follow proper casing. Opening tag & its 
corresponding closing tag must be written in the same way. 
e.g. <NAME>ABC</NAME> is well formed. Whereas 
<NAME>ABC</Name> is not well-formed.

If XML is confirming to DTD rules then it's a valid XML.
If a XML document is conforming to XML rules (all tags 
started are closed,there is a root element etc)then it's a 
well formed XML.