Is it legal for my employer to keep my passport in Kuwait?

Your passport belongs to you. Employers taking and holding a worker's passport against their will is not allowed and is treated as a form of control. If your employer has taken your passport and will not return it, first ask for it back in writing or by message, so you have a record. If they still refuse, report it to the Public Authority for Manpower (PAM) on 103, or tell the Bangladesh Embassy labour wing, which can take it up with the authorities. Keep copies of your passport photo page and visa in your phone and somewhere safe, separately from the original, so you always have your details even if the original is held.