According to the system from the US, the ambassadors are either career diplomats (Foreign Service Officers), or are politically appointed, on the branch of the US President or of the party from which he originates. All the ambassadors serve "at the discretion of the president," meaning they can be revoked at any time by the US President, for the appointment being needed the approval of Congress.
James Pettit is a career diplomat for over 30 years. He was appointed in Moldova in 2015. Over time, the official occupied important positions in the US consulate in Kiev, Moscow and Vienna.
The elected president, Donald Trump's decision to recall the ambassadors appointed by Barack Obama refers to politically appointed ambassadors, not career diplomats. Usually, it is about the ambassadors in Moscow, London, Berlin, Tel Aviv, Tokyo, Paris and other major capitals.
Thus, Pettit would not be affected by this measure, have explained the diplomatic sources, but he also, like other ambassadors, can be removed by a direct decision of the US President.