There are many Harvard microcontrollers, DSPs are mostly Harvard, but the key advantages of DSP are:
- most instuctions are single cycle, and 16 or 32 bit devices;
- supports special instructions for signal processing: multiplications, multiply and accumulate the result (MAC) in 1 cycle, data adjusting, representation conversion;
- special addressing modes, circular buffers needed for filter implementation along with MAC, bit reversed addressing needed for FFT;
- DMA.
A few uC are also singlecycle and are 16b, but without the DSP features it takes a long time to move data back and forth in software, so they can't be used as DSP for a resonable complex project.