ContourPlot3D[x^2+y^2+x*y==z,{x,-2,2},{y,-2,2},{z,0,5}, MeshFunctions->{#3&}]

ContourPlot3D[x^2+y^2==z,{x,-2,2},{y,-2,2},{z,0,5}, MeshFunctions->{#3&}, AxesLabel->{x,y,z}]

ContourPlot3D[x^2+y^2+2*x*y==z,{x,-5,5},{y,-5,5},{z,0,10}, MeshFunctions->{#3&}, AxesLabel->{x,y,z}]

ContourPlot[x^2+y^2+2*x*y,{x,-5,5},{y,-5,5}]

ContourPlot3D[x^2+y^2+3*x*y==z,{x,-5,5},{y,-5,5},{z,-10,10}, MeshFunctions->{#3&}, AxesLabel->{x,y,z}]

ContourPlot3D[x^2+y^2-3*x*y==z,{x,-5,5},{y,-5,5},{z,-10,10}, MeshFunctions->{#3&}, AxesLabel->{x,y,z}]

ContourPlot3D[x^2+y^2-2*x*y==z,{x,-5,5},{y,-5,5},{z,0,10}, MeshFunctions->{#3&}, AxesLabel->{x,y,z}]