Prepare pasta according to package directions in a pot of salted boiling water, adding shrimp and peas to water during final 2 minutes of cook time. Drain pasta mixture, reserving 1/2 cup cooking water in a small heatproof bowl. Return pasta mixture to pot.

Add pesto, salt, and pepper to pasta mixture; stir to combine. Stir in reserved cooking water as needed, 1/4 cup at a time, until desired consistency is reached. Transfer pasta mixture to a serving bowl; sprinkle with Parmesan. Serve immediately.