There's one man who definitely wouldn't be welcome at this week's summit. In fact, if Maj. Stanislav Levchenko were to show up anywhere near the Reagan-Gorbachev pageantry, he would be risking his neck. Forget glasnost, detente, nuclear disarmament and Mikhail Gorbachev's sunny smile. The Kremlin's KGB agents would do their utmost to grab Levchenko and smuggle him back to Moscow, where he would be summarily executed.

Why? Levchenko used to be one of the KGB's most effective agents -- skillful at planting disinformation with witting or unwitting editors, positively brilliant at luring susceptible foreign nationals into working for the KGB -- in short, an all-around good ole Kremlin spook.

But in October 1979, Levchenko defected in Tokyo, sending shock waves through the KGB. In August 1981, a secret military tribunal in Moscow found him guilty of high treason and sentenced him, in absentia, to the ''highest measure of punishment.''

Even with Gorbachev's more benign glasnost, Levchenko is under no illusions. He knows his life isn't worth a plugged kopeck if the KGB ever gets its hands on him. During a series of clandestine meetings with Dale Van Atta, Levchenko never disclosed where he's living. He travels under a pseudonym -- and after eight years still drives with an eye on the rear-view mirror, looking for a KGB tail.

This is no more than reasonable precaution. We have been told that one particular ''diplomat'' at the Soviet Embassy in Washington has one and only one mission: locate and snatch Levchenko. During the trial of Richard Miller, the only FBI agent ever convicted of espionage, witnesses testified that Levchenko headed the KGB's list of most-wanted Soviet defectors. Witnesses said that Miller's Soviet handlers asked him to check with his FBI buddies and find out where Levchenko was hiding out.

''Few men understand the KGB better than Levchenko,'' wrote Readers Digest senior editor John Barron in his book, ''KGB Today: The Hidden Hand.'' Barron, the most knowledgeable American author on the Soviet secret service, concluded: ''{And}few, if any, have done as much as Stanislav Alexandrovich Levchenko to wound the KGB. He understands that, as a consequence, he will always be a hunted man.''

Levchenko, now 46, managed to hide the fact that his mother, who died in a later childbirth, was Jewish, by being adopted by his stepmother. His father was a research chemist and army officer.

In 1958, the teen-age Levchenko began six years of training at Moscow's Institute of Oriental Languages, and became fluent in both English and Japanese. In 1966 he was drafted into the GRU, Soviet military intelligence, and was trained for a World War III suicide mission to Liverpool.

In 1968, against his wishes, he was recruited by the KGB, the civilian spy outfit. Eventually he was placed on the staff of the magazine Novoye Vremya, or New Times, which is riddled with KGB agents. Under the magazine's journalistic cover he was sent to Tokyo as a spy in 1975.

Levchenko scored many espionage coups for his KGB masters while he was in Tokyo. They included early warning of the Lockheed bribery scandal, which shook the Japanese government to its roots, and recruitment of a Japanese intelligence official who gave the KGB total access to secret files, including minutes of a conference in which top Japanese intelligence officials discussed their operations against the Soviet Union, and the 700-page directory of all Japanese security officers -- including their names, addresses and telephone numbers.

While he was accomplishing these Herculean feats of espionage, however, Levchenko had secretly become a Christian, and had grown disenchanted with the Soviet system. Yet, when he defected and was taken over by the CIA debriefers, he provided only minimal information at first.

Two telephone calls to Moscow in 1980 changed that. Both were to his wife, Natalia, whom the KGB had thrown out of work and nearly starved to death. Levchenko's son had been ostracized at school, and had developed high blood pressure and a stomach disorder.

On the second call, Levchenko's wife told him, in a faint voice, that she had shrunk from 120 to 90 pounds. ''I am without hope,'' she said. ''I am lost. I am only a grain of sand.''

That did it. Levchenko declared war on the KGB that day. He briefed the CIA 12 hours a day, seven days a week. He unraveled the KGB's entire Tokyo spy network, painstakingly built up over decades.

He fingered the KGB's 25 top agents in Japan, and with his incredible memory for names also identified hundreds of KGB spies and the people they had recruited in Western countries. Some of his analyses have been read personally by President Reagan.

''Maybe someday they will find me,'' Levchenko said with classic Russian fatalism. ''But I must continue to fight them. If I do not -- if we do not -- they will degrade all human beings into simple grains of sand.''