What Validates the Xml Document Against the Schema/Dtd?

An XML document with correct syntax is called "Well Formed". An XML document validated against a DTD is both "Well Formed" and "Valid".

What validates the XML document against the schema?

You can validate your XML documents against XML schemas only; validation against DTDs is not supported. ... This process involves registering each XML schema document that makes up the XML schema and then completing the registration.

Which engine validates the XML document against the schema DTD?

Xerces-C++ is an open-source validating XML parser written in a portable subset of C++. It provides DOM (level 1, 2, and certain parts of level 3), SAX, and SAX2 APIs and supports validation of XML documents against DTD and XML Schema.