Working over a shallow bowl, cut off corn kernels, then scrape each cob with a knife to extract juice. Discard cobs.
Puree 2 cups kernels, reserving remainder, and all of juice in a food processor 2 minutes, scraping down sides. Force through a fine sieve into a bowl and discard solids. Stir in salt, cornstarch, and sugar.
Cook onion in butter in a saucepan over moderately low heat, stirring frequently, until softened. Add reserved kernels and water and simmer briskly, covered, stirring occasionally, until corn is crisp-tender, 4 to 5 minutes.
Stir corn puree again, then stir into kernels. Bring to a boil, stirring, and simmer, stirring, until thickened, about 2 minutes. (If desired, thin with water.) Season with pepper and stir in chives.