Speech levels are different.
안녕하십니까 is a Formal, highly addressee-raising form.
안녕하세요 is an Informal, addressee-raising form.
안녕 is an Informal, addressee-lowering form.
Korean has a complex honorific system. it can be classified into formal/informal form and sorted again by honorific degree like below(roughly, honorific/non-honorific form).
Formal, highly addressee-raising : 합쇼체
Formal, moderately addressee-raising : 하오체
Formal, moderately addressee-lowering : 하게체
Formal, highly addressee-lowering : 해라체
Informal, addressee-raising : 해요체
Informal, addressee-lowering : 해체
Nowadays formal forms except 합쇼체 are rarely used in daily conversation. But still you can hear them from a very limited situation or from k-drama. Especially you can hear all types of speech level in a period drama.
And this system is not obeyed strictly. the styles are often mixed in a real conversation.
When you learn Korean first, it is enough to practice two informal forms.
If you want to know usage more, examples for each style are here. Carefully watch the situations and the relationships between characters.
References are from k-drama series 'Kingdom'.
Kingdom | Featurette : The Making of Kingdom: https://youtu.be/GXyXqTAYhJk
Kingdom | Official Trailer #2 : https://youtu.be/Bmhqw0F3q-g
Kingdom | Official Trailer : https://youtu.be/4l-yByZpaaM
Kingdom Season 2 | Main Trailer : https://youtu.be/7ecrESKwh_Y
○합쇼체(하십시오체) Formal, highly addressee-raisinghttps://youtu.be/4l-yByZpaaM?t=45
“죽은 사람들이 괴물이 되어 살아났습니다. 이제 곧 깨어납니다.”
(The dead people came back to life as monsters. They will wake up soon.)https://youtu.be/4l-yByZpaaM?t=91
“조학주의 만행이 극에 달했습니다. 도와주십시오.”
(Cho Hak-ju is as atrocious as ever. You must help.)https://youtu.be/7ecrESKwh_Y?t=74
“아무리 끔찍한 병도 막을 방도가 있었습니다.”
(However severe the disease, there was always a way to stop it.)https://youtu.be/7ecrESKwh_Y?t=105
“둘 중 하나는 죽어야 합니다.”
(One of us must die.)
In present day, 합쇼체 still widely used at workplace and in public speech.
○하오체 Formal, moderately addressee-raisinghttps://youtu.be/4l-yByZpaaM?t=22
“강녕전에서 몰골이 끔찍한 괴물을 보았소. 아바마마께서는 어디 계시오?”
(I saw a hideous monster inside the King's palace. Where is my father?)https://youtu.be/7ecrESKwh_Y?t=92
(Step down from the throne.)
Crown Prince usually speaks with 합쇼체 to them(high ranked bureaucrat, queen) as you saw above examples. but now Crown Prince lowers speech level to urge something.
Nowadays this style is not used in normal conversation, but sometimes used in lyrics because it sounds poetic. '편지(letter)' by Kim Gwang-jin is famous.
○하게체 Formal, moderately addressee-lowering
I cannot find this style in trailers.
This style also rarely used these days. Occasionally an old gentleman like professors or parents-in-laws say in this way.
○해라체 Formal, highly addressee-loweringhttps://youtu.be/7ecrESKwh_Y?t=35
"궁궐의 모든 문을 걸어 잠가라!”
(Lock up all the gates at the palace!)https://youtu.be/7ecrESKwh_Y?t=45
“나도 저것들을 이용해 권세를 누릴 것이다.”
(I will use them to gain more power.)https://youtu.be/7ecrESKwh_Y?t=51
“내가 만들려는 나라는 그런 나라가 아니다.”
(That is not the kind of country I am trying to build.)https://youtu.be/7ecrESKwh_Y?t=58
“나는 내가 있을 자리로 돌아갈 것이다. 나와 함께 하겠느냐?”
(I will return to where I rightfully should be. Will you join hands with me?)https://youtu.be/7ecrESKwh_Y?t=88
“저 놈을 당장 잡아오너라!”
(Arrest that bastard at once!)
Since this form sounds too formal & unfriendly, you cannot hear in daily conversation. But, still you may hear this in military base like Korean meme “중대장은 너희에게 실망했다(you disappointed me).” if a general speaks to soldiers in an informal way, it may sound unauthoritative.
●해요체 Informal, addressee-raisinghttps://youtu.be/Bmhqw0F3q-g?t=28
(Don't abandon us!)https://youtu.be/4l-yByZpaaM?t=70
(What about the Crown Prince?)
It is hard to hear 해요체 in the acting scenes from trailers. But you will easily find actors & producers usually use 해요체 when they are interviewed(https://youtu.be/GXyXqTAYhJk
). it sounds more friendly than any formal forms.
●해체 Informal, addressee-loweringhttps://youtu.be/Bmhqw0F3q-g?t=14
“어떻게 사람이 사람을 먹어?”
(How can humans eat humans?)
You can use this to your close friends or kids. Since you can be considered being rude, you need to use this form carefully.
☆하소서체 extremely addressee-raisinghttps://youtu.be/4l-yByZpaaM?t=37
“여기있는 사람들에게 아무래도 무슨 큰 일이 벌어진 것 같사옵니다.”
(I think something terrible happened to the people here.)
This style is not used in modern days, because it is spoken to the royal family or to the god. You may only hear 하소서체 from prayer in church.