Harvard architecture has separate data and instruction busses, allowing transfers to be performed simultaneously on both busses.

A von Neumann architecture has only one bus which is used for both data transfers and instruction fetches.

therefore data transfers and instruction fetches must be scheduled – they can not be performed at the same time. but It is possible to have two separate memory systems for a Harvard architecture. 

Following are the difference between harvard architecture and von-neumann architecture

Harward Architecture	Von Neumann Architecture
The name is originated from “Harvard Mark I” a relay based old computer.	It is named after the mathematician and early computer scientist John Von Neumann.
It required two memories for their instruction and data.	It required only one memory for their instruction and data.
Design of Harvard architecture is complicated.	Design of the von Neumann architecture is simple.
Harvard architecture is required separate bus for instruction and data.	Von Neumann architecture is required only one bus for instruction and data.
Processor can complete an instruction in one cycle	Processor needs two clock cycles to complete an instruction.
Easier to pipeline, so high performance can be achieve.	Low performance as compared to Harvard architecture.
Comparatively high cost.	It is cheaper.