  We develop a new refinement of the Kato's inequality and using this
refinement we obtain several upper bounds for the numerical radius of a bounded
linear operator as well as the product of operators, which improve the well
known existing bounds. Further, we obtain a necessary and sufficient condition
for the positivity of $2\times 2$ certain block matrices and using this
condition we deduce an upper bound for the numerical radius involving a
contraction operator. Furthermore, we study the Schatten $p$-norm inequalities
for the sum of two $n\times n$ complex matrices via singular values and from
the inequalities we obtain the $p$-numerical radius and the classical numerical
radius bounds. We show that for every $p>0$, the $p$-numerical radius
$w_p(\cdot): \mathcal{M}_n(\mathbb C)\to \mathbb R$ satisfies $ w_p(T) \leq
\frac12 \sqrt{\left\| |T|^{2(1-t)}+|T^*|^{2(1-t)} \right\|^{} \, \big
\||T|^{2t}+|T^*|^{2t} \big\|_{p/2}^{} } $ for all $t\in [0,1]$. Considering
$p\to \infty$, we get a nice refinement of the well known classical numerical
radius bound $w(T) \leq \sqrt{\frac12 \left\| T^*T+TT^* \right \|}.$
  As an application of the Schatten $p$-norm inequalities we develop a bound
for the energy of graph. We show that
  $ \mathcal{E}(G) \geq \frac{2m}{ \sqrt{ \max_{1\leq i \leq n} \left\{
\sum_{j, v_i \sim v_j}d_j\right\}} },$ where $\mathcal{E}(G)$ is the energy of
a simple graph $G$ with $m$ edges and $n$ vertices $v_1,v_2,\ldots,v_n$ such
that degree of $v_i$ is $d_i$ for each $i=1,2,\ldots,n.$
