You can use:
привет - privet - "hi" for your friends (agemates), parents
здравствуй - zdrastvuj - "hi" for little children or said by old people
здравствуйте - zdrastvujte - "hi" for old people, official
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здорово - zdarova - "hi" using with old friends, if the comisng is unexpected, etc.
хай - haj - barbarism from Englsih popular with youth