Shahan is a Persian name for boys (also used for girls, but rarely) that means "king-like", "exalted", "royal", "worthy of a king". It is not an Islamic name but Muslims can use it since it has a good meaning. It is written as شاهان in Farsi and can also be written as Shahaan, Shaahan in English. It is pronounced [(SH)op + m(A)n] + [(HAN)d] with emphasis on the first syllable.
Shahana is the girl name version of Shahan and has the same meaning. It is pronounced [(SH)op + m(A)n] + [(HA)] + [(NU)mber or (NE)ck or (NA)p] and is written as شاهانه in Farsi. It can be written as Shahaana, Shahaneh in English.
For Shayan please see this answer: What is the meaning of Shayan?