What is the difference between value types and primitive types?

A value type is usually whatever type reside on the Stack .

A primitive type is a type defined at the programming language level, often it is even a value type, directly supported by the compiler of the language.

However this is a summary general answer because each programming language have different set of differences between the two types ...

A primitive type (e.g. int) can be mapped directly to a Base Class Library (BCL) type (e.g. System.Int32)

A value type inherits from System.ValueType and is passed by value (among other properties).

They are not interchangeable as object (System.Object) is a primitive type but not a value type, and structs are value types but not primitive.

See more differences here