안녕하세요
안녕하십니까
안녕

Ciao can be mean Hello informally(Or formally) for Greeting & Goodbye informally for Farewell.

안녕 (informal)
ㅎㅇ (하이) (Slang & Texting, informal / casual)
안녕하세요 (formal / polite)
안녕하십니까 (honorific / very polite)

안녕 is informal or casual used to the same age(friend), someone younger than you, the children, lover
(low formality / low politeness)

안녕하세요 is 세 is honorific + 요 is informal (but still) polite used to strangers(someone you don't know), olders, seniors, teacher, boss, everyone
(low formality / high politeness)

안녕하십니까 is honorific or very polite used to olders, elders, seniors, boss, army, officers, everyone.
(high formality / high politeness)
(Ciao/Hello/Hi for Greeting)

안녕 (informal / casual)
(Farewell)
When you stay / when someone leaves→ 잘 가 (informal / casual)
When you leave / when someone stays→ 잘 있어 (informal / casual)
잘 가(세)요 (informal polite & 세 => honorific)
안녕히 계세요 (more formal) polite = 잘 있어
안녕히 가세요 (more formal) polite = 잘 가
(Bye for Farewell)