Thomas Krens has been appointed director of the Guggenheim Museum in New York and the Peggy Guggenheim Collection in Venice, it was announced Wednesday. Peter Lawson-Johnson, president of the Solomon R. Guggenheim Foundation, said Krens, 41, would succeed Thomas M. Messer, who will retire June 30 after 27 years as museum director and director of the Solomon R. Guggenheim Foundation, which operates the museum. Krens, who served as director of the Williams College Museum in Williamstown, Mass., also will become director of the foundation. Krens will continue to teach a course in contemporary and 20th-Century art as well as heading the planning group for the proposed Massachusetts Museum of Contemporary Art at North Adams, Mass. The New York City native joined the art department faculty of Williams in 1971 and became director of the college museum in 1980.