The tags of hibernate .cfg. XML are DTD, JDBC connection, and SQL variant to generate. DTD simply stands for Document Type Definition. It is a kind of markup declarations that define a type of document for SGML family markup language (SGML, HTML, GML, XML).
A DTD defines the building blocks that are valid for an XML document. It defines the structure of the document providing a list of validated attributes and elements. JDBC simply stands for Java Database Connectivity. It is an API (application programming interface) for Java programming language that defines the way a client might actually access a database.
It is a data access technology that is Java-based and used for Java database connectivity. For the SQL variant, you can use it in variables, parameters, columns, user-defined functions' return values. SQL variant enables support between these database objects and the values of other data types.