DMV in SQL Server is a catch-all term for Database Management Views [& Functions]. Technically they are all now DMOs (Database Management Objects) however the term DMV is still widely understood and accepted by most.
Database Management Views & Functions began being added to SQL Server starting with the 2005 version. They are meant to provie a consistent way of retrieving system, database, performance, etc... information out of SQL Server. Several of the DMVs are direct replacements for DBCC commands so that you can join the resulting information returned with another table or query.
Resource: For more information on Database Management Views, Functions, Objects see: http://msdn.microsoft.com/en-us/library/ms188754.aspx
