Yes, "Hi" is more casual than "Hello", but it's not quite as casual as Привет seems to be. For example, an English speaker can say "Hi" to his/her boss even though there's probably still some formality between the two people. "Hey" (used between friends and close acquaintances) might be the closer equivalent to Привет.
(In personal emails and chats, I've also used "Hey you" or "Heya" (which is my own shorthand for "Hey ya" though I'm not sure if anyone else uses it)).
i found this in internet :
Здравствуйте is plurar form of the verb здравствуй. In russian verbs can be plural.
In case of здравствуйте it means to greet a group of people. Also you greet a single person with plural здравствуйте to express respect. So you don’t greet an older person with singular здравствуй, this would be very rude.
And you don’t usually greet a child with plural здравствуйте, this is rather humourous (group of children - yes). And you never greet a group of people with singular здравствуй, this is grammatically incorrect.
I know this is really late, but Привет is your standard hello when greeting friends. Алло is when you answer the phone. This is very common in other languages as well (IE. Spanish, you say Alo instead of Hola when answering phone). Use Здравствуйте as the most formal with people you would address as Вы. Здравствуй and Привет are both for people you address with Ты, but Привет is the most commonly used one. This is my understanding. I'm not a native speaker