The Torah states that the angels ate the food which Avraham served them, on which Rashi comments: "they pretended to eat".
It appears, therefore, that Avraham did not perform a mitzvah by feeding his guests, for ultimately they were angels who did not need to eat. Thus: a) Avraham interrupted his audience with the Divine Presence unnecessarily, and b) How can we learn the principle that "welcoming guests is greater than welcoming the Divine Presence" [Shabbos 127a] from an incident where there were no real guests present?
Generally speaking, with acts of kindness, the primary focus is on the results of the mitzvah, i.e. the benefits given to the guests - food and drink etc.
The unique quality of the mitzvah of welcoming guests is that the primary focus is not on the benefits received by the guests, but rather on the good will demonstrated by the host. In this light, it turns out that Avraham did fulfill the mitzvah of welcoming guests, in the most exemplary manner.
Based on Likutei Sichos of the Lubavitcher Rebbe, Gutnick Chumash