What is the difference between Von Neumann and Harvard architectures?

Von Neumann architecture uses single memory and bus for both instructions and data, allowing simpler design but creating a bottleneck (can only fetch instruction OR data per cycle). Harvard architecture uses separate memories and buses for instructions and data, enabling simultaneous instruction fetch and data access for higher performance. Most modern microcontrollers use modified Harvard (separate memories but can access code as data). DSPs often use Harvard for maximum throughput.