XmlDocument and XPathDocument are based on different data 
models.
XmlDocument is based on W3C XML DOM, which is object model 
that
basically covers all XML syntax, including low level syntax 
sugar such
as entities, CDATA sections, DTD, notations etc. That's 
document-centric
model and it allows for full fidelity when loading/saving 
XML documents.

XPathDocument is based on XPath 1.0 data model which is 
read-only XML
Infoset-compatible data-centric object model that covers 
only
semantically significant parts of XML, leaving out 
insignificant syntax
details - no DTD, no entities, no CDATA, no adjacent text 
nodes, only
significant data expressed as a tree with seven types of 
nodes. Simple
and lightweight.

mainly it is a part of xsl ,it is a language that is used to
acess different parts of an xml document, such as element
and attributes.

mainly it is a part of xsl ,it is a language that is used to
acess different parts of an xml document, such as element
and attributes.

xml document usually begins with the xml declaration
statement also called PI(processing instruction).