They'd go splat. Pathfinder monsters have higher hit points and do more damage than 3E monsters which, in turn, had higher hit points and do more damage than OSR critters (including S&W) tend to.
Eyeball a few of the monsters that appear in both systems. My guess is you're looking at a single level difference at the lowest end and maybe up to five levels at the highest.
If you don't feel like converting all the monsters in an AP (which is what I'm guessing is the root of this question), I'd either use the S&W stats for all monsters that exist for it, look for an existing conversion (I bet they exist) or just start the AP with the characters at level 3 or so.
The escalation dice helps both sides equally. So they just splat faster.
And yes, I'd get identical monsters from each edition and compare total hit points and damage for each to estimate the level difference. Honestly, a good monster compendium like Monstrosities is your best bet here.
But at tenth level, things get way out of whack.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.