The long-delayed renovation of the historic Willard Hotel, the cornerstone of efforts to revitalize Pennsylvania Avenue, will begin on Dec. 29, according to the new chairman of the agency supervising the redevelopment of the avenue.

Henry A. Berliner Jr., a lawyer and businessman who was sworn in yesterday as chairman of the Pennsylvania Avenue Development Corporation (PADC), said he has made completion of the Willard project within the next two years his top priority.

"The project is going to go forward," he said.

Berliner, who was appointed by President Reagan to replace Max N. Berry as chairman, said he was confident that developer Oliver T. Carr Jr. would begin working on the project on time--including preliminary demolition work in July.

However, Berliner warned, during a meeting of the PADC directors, that he would seek to reopen the project to bidding by other contractors if Carr misses his deadlines.

Carr's firm was granted control of the $90 million project by the PADC in October 1981, after Florida businessman Stuart S. Golding, the original developer, spent three frustrating years trying to arrange the financing.

Carr said late yesterday that "everything is very much in order," and that he would have no problem in meeting the deadline.

"We're pleased with our relationship with the board of the PADC and we only look for good things to happen in the future," he said.

In winning control of the project, Carr pledged to obtain hard-to-come-by long-term financing for the renovation in return for a major say in revising the plans for the hotel and adjoining commercial property on the northwest corner of Pennsylvania Avenue and 14th Street NW.

The PADC went along with Carr's plans to reduce the number of hotel rooms in the project from 600 to 350 and to add a total of 225,000 square feet of commercial and office space.

However, the D.C. government recently rejected a request from Carr to revise plans to permit cars to enter a hotel underground garage from Pennsylvania Avenue, instead of F Street.

Carr said yesterday that he has arranged for a short-term construction loan for the project, but declined to discuss details of his arrangement for long-term financing.