I love you.
"It's me who loves you." This would be said in the context where you're contrasting with someone else:
"He likes you, but I love you!"
I love you.
You're emphasizing the fact that it's love, rather than anything else. This is also the emphasis you would use if you want to emphasize the whole sentence.
For example:
"I'm moving to South Africa!"
"What?! You can't! I love you!"
I love you.
It's you that I love. Again, contrasting with another person, but it's the loved person who's being contrasted now, not the one who is doing the loving.
"Of course there's no-one else. I love you, silly."
I love you very much.
Here you're just emphasizing the degree of the love.
Other ways of verbally emphasizing "I love you" to express different meanings
I can think of a couple:
I do love you.
This "do" is a common way of expressing emphasis. Read more in this answer.
I love you.
This can be a way of saying "I love you too" if it comes after "I love you".
"I love you."
"And I love you."
This emphasis could come after "Bob loves Tom" or something, it doesn't have to come after "I love you".