Basically DMA called is direct memory access can as it sounds can make transfers directly to memory. PIO mode stands for programmed I/O requires the CPU to get involved, taking CPU cycles. The problem with many zorro II DMA cards is they can only "DMA" to 16bit memory on a A3000 or A4000 the 32bit memory is at a different location. Some zorro II cards can DMA to 32 bit memory in an a3000 or a4000 but the transfer is only 16bit and can slow things down in other ways.
I have a habit having some 16bit fast ram, 2megs or more, in my a2000s and a500s even if I have a 32bit accelerator cards. This aids in DMA transfers on their respective the 16 bit buses. On A3000s or a4000s it is a bit of different story.
The A4091 is a zorro III card and can used the 32 bit bus.
I am still a little confused at how and when the Amiga made use of PIO versus DMA, specifically with SCSI and IDE configurations. I remember it was always a big deal that the Amiga had DMA channels
