Answer
$4uv$
Work Step by Step
$x^{2}-y^{2}=\left( x-y\right) \left( x+y\right) \Rightarrow$
$ \left( u+v\right) ^{2}-\left( u-v\right) ^{2}=\left( \left( u+v\right) -\left( u-v\right) \right) \left( \left( u+v\right) +\left( u-v\right) \right) =2v\times 2u=4uv$