adverb
Used to emphasize that something appears or seems a certain way, often with a nuance of 'indeed' or 'very much so'. Common before verbs like そうだ, らしい, 困った, 嬉しそう, etc.
彼はさも嬉しそうに笑った。
He smiled as if he were truly happy.
さも当然のように彼女は答えた。
She answered as if it were the most natural thing in the world.
adverb
Archaic or literary use meaning 'in that way' or 'thus'. Mostly found in the set phrase さもありなん (it may well be so; that's quite possible).
See also: さもありなん
「さもありなん」という古い言い回しがある。
There is an old expression 'さもありなん' (it may well be so).
いかにも also means 'truly' or 'indeed', but is more emphatic and often used in exclamations or to express strong agreement. さも is more about describing appearances or manner.
まるで means 'as if' or 'completely', often used in similes. さも focuses on the seeming reality of a situation rather than a comparison.
Derived from classical Japanese 然 (sa, 'that way') + も (mo, 'also/even'). The kanji 然 is used for its meaning 'thus' or 'in that way'.