A tree is perfectly balanced if it is empty or the number of nodes in each subtree differ by no more than 1. In a perfectly balanced tree, we know that searching either the left or right subtree from any point will take the same amount of time.
The search time in a perfectly balanced tree is O(log n) as the number of nodes left to consider is effectively halfed with each node considered. However, to get a tree to be perfectly balance can require changing every node in the tree. This makes trying to create a perfectly balanced tree impractical.
An AVL tree does not create a perfectly balanced binary search trees. Instead it creates a height balanced binary search trees. A height balanced tree is either empty or the height of the left and right subtrees differ by no more than 1. A height balanced tree is at most 44% taller than a perfectly balanced tree and thus, a search through a height balanced tree is O(log n). Insert and delete can also be done in O(log n) time.