In a covered 5- to 6-quart pan over high heat, bring about 3 quarts water to a boil.
Meanwhile, in a 10- to 12-inch frying pan over medium heat, frequently stir oil, onion, and garlic until onion is limp, about 5 minutes.
Add cream and tomatoes (with juice) to frying pan. Bring to a boil over high heat. Add shrimp and stir often until pink, 3 to 4 minutes. Add parsley and salt and pepper to taste. Remove from heat and keep warm.
When shrimp is in pan, add fettuccine to boiling water and cook just until pasta is tender to bite, 3 to 4 minutes. Drain well; return pasta to pan.
Stir shrimp mixture with pasta.