You can easily push them back if you 2 gate as soon as you see this coming, pull probes, and have individual zealots chase individual marines, and 2-3 probes chase individual marines. Ignore SCV's, they won't do much damage at all. If the marines stop to fire they get raped and the SCV's just run around chasing your probes and zealots not doing much at all.
It turns into a chase game until one of the players gets overpowered by better micro.
The easiest counter to this is the faster realization that he is doing this rush by really good scouting.