In American English, the past tense and past participle of dwell is usually dwelled. In varieties of English from outside North America, dwelt is the preferred form. Both are common in Canadian English.
Both forms are many centuries old, but dwelt has been more common for at least three centuries. The American preference for dwelled is a new development. A Google Ngram charting the words’ use in 20th-century American books and magazines shows dwelt still prevailing at the end of the century, but this isn’t borne out everywhere. In web searches limited to American news sources, the ratio of dwelled to dwelt is about 1:1 in the 1980s, 2:1 in the 1990s, 2:1 in the 2000s, and 3:1 since 2010.
Dwelt is the standard form outside North America:
Once, writers dwelt quite apart from their readers. [Guardian]
The rain came down and in the crowd of 62,989 neutral thoughts might have dwelt on the pathetic fallacy. [Irish Times]
Those who supported the budget dwelt on the National Disability Insurance Scheme. [The Australian]
And though dwelt still appears occasionally in American sources, examples such as these are much easier to find:
Tuesday’s City Council discussion dwelled heavily on the problem of getting citizens to cooperate with police. [Los Angeles Times]
You know that you have dwelled in Washington too long when you begin to have the creeping sense that you are Important. [Washington Post]
He dwelled on the fact that consumers don’t like or trust wireless carriers. [New York Times]