Differently from the Neumann architecture, The Harvard architecture has different storage and buses for code instructions and data.
Modern processors appear to the user to be systems with von Neumann architectures, with the program code stored in the same main memory as the data. For performance reasons, internally and largely invisible to the user, most designs have separate processor caches for the instructions and data, with separate pathways into the processor for each. This is one form of what is known as the modified Harvard architecture.
An important note is that:
A modified Harvard architecture is a variation of the Harvard computer architecture that, unlike the pure Harvard architecture, allows memory that contains instructions to be accessed as data. Most modern computers that are documented as Harvard architecture are, in fact, modified Harvard architecture.
This modification is widespread in modern processors, such as the ARM architecture, Power ISA and x86 processors. It is sometimes loosely called a Harvard architecture, overlooking the fact that it is actually "modified".
Harvard architecture model is mainly used in microcontrollers: microcontrollers have a small program and data memory that needs to be accessed simultaneously. However, Harvard architecture is not always used in microcontrollers; some microcontroller families use hybrid or Von Neumann architecture models.