A funny Question~

The logician Raymond Smullyan describes an island containing two types of people:

Knights who always tell the truth,

Knaves who always lie.

The condition is: You are visiting the island and you encounters withe natives.

You meet a group of six natives, U, V, W, X, Y, Z, who speak to you as follows:

U says: None of us is a knight.

V says: At least three of us is a knights.

W says: At most three of us are knights.

X says: Exactly five of us are knights.

Y says: Exactly two of us are knights.

Z says: Exactly one of us is a knight.

Which are knights and which are knaves???