Does pyruvic acid enter the Krebs' cycle directly?



No, before pyruvic acid enters Krebs’ cycle, operative in the mitochondria, one of the three carbon atoms of pyruvic acid is oxidized to carbon dioxide in a reaction called oxidative decarboxylation, that is, pyruvate is first decarboxylated, and then oxidized by the enzyme pyruvate dehydrogenase. The combination of the remaining 2-carbon acetate unit is readily accepted by a (Co-A), to form acetyl Co-A. This is the connecting link between glycolysis and Krebs’ cycle. During the process, ${{NAD}^{+}}$ is reduced to NADH + ${{H}^{+}}$.
The summary of the reaction is given below

During this process, two molecules of NADH are produced (from the metabolism of two molecules of pyruvic acid produced during glycolysis), and thus, it results in a net gain of 6 ATP molecules (2NADH 3 = 6 ATP).