Khenchen means "Senior Khenpo". The Kagyu, Nyingma and Sakyapa schools use the title of Khenpo for those who have achieved the highest levels of mastery in Shedras. Why? The reason is that in Tibet, the teachers who achieved such mastery also were the ones who gave the monks vows.
khenpo; Abbot or chief instructor or spiritual authority of a monastery. The title is also accorded to lamas of great learning.
However, the term for a teacher who has received the highest levels of teachings is Lob Pon. This title essentially means "teacher." The senior teacher, or principal, is called Lob Chi. One can be a very good philosophical teacher, or Lobpon, without receiving the full monk's vows. Teachers who have kept the monk's vows very well, and achieved maturity, can be appointed Khenpo— or "giver of monk's vows." But not until they have proven able to keep the monk's vows can a teacher be appointed Khenpo. The title of Khenpo and Khenchen should only be used by those who have demonstrated the highest vinaya conduct and have become very accomplished in this respect.
Over time, the term Khenpo has come to refer loosely to the teachers who receive the highest levels of scholarship in the shedra.
Khenchen: The younger son in Tibetan lineages while the eldest is the Trichen.