There are two types of pressure system pumps that don't need accumulator tanks.
Jabsco makes variable speed pumps that will run just fast enough to supply your need. If you open the faucet just a little, it runs slow. Open the faucet more and it speeds up. This is the Vflo series of pumps.
ShurFlo stopped making variable speed pumps and went to bypass type pumps.
When you open a faucet with a ShurFlo, the pump comes on full speed just like a pressure switch pump. If the faucet isn't open enough to handle the full flow, the excess water is shunted back to the intake of the pump and is pushed back to the tank. This is a simpler system, but noisier, than the variable speed as the pump is on full speed when ever water is drawn. This is the ShurFlo Aqua King II series of pumps.
AQUA KING™ II Fresh Water Pumps