Plot3D[
      Sqrt[2*x^2 + y^2 - 3], {x, -4, 4}, {y, -6, 6}]

ParametricPlot3D[{Sqrt[(3 + z^2)/(1 + Cos[phi]^2)]*
          Cos[phi], Sqrt[(3 + z^2)/(1 + Cos[phi]^2)]*Sin[phi], 
        z}, {phi, 0, 2*Pi}, {z, 0, 6}]

ContourPlot[
      Sqrt[2*x^2 + y^2 - 3], {x, -6, 6}, {y, -6, 
        6}]