extend vs lengthen

extend and lengthen both are verbs.

Word	Noun	Adjective	Verb	Adverb
extend	No	No	Yes	No
lengthen	No	No	Yes	No

As verbs, lengthen is a hypernym of extend; that is, lengthen is a word with a broader meaning than extend:

extend (verb)	lengthen (verb)
extend one's limbs or muscles, or the entire body	become long or longer
thrust or extend out	make longer
extend in scope or range or area
lengthen in time; cause to be or last longer
continue or extend
expand the influence of
increase in quantity or bulk by adding a cheaper substance
prolong the time allowed for payment of
use to the utmost; exert vigorously or to full capacity
open or straighten out; unbend
extend or stretch out to a greater or the full length
cause to move at full gallop
offer verbally
make available; provide
span an interval of distance, space or time
reach outward in space
stretch out over a distance, space, time, or scope; run or extend between two points or beyond a certain point