How to increase space between tabulars inside a table

(re-posting my earlier comment as an answer, so that this query can be considered to have received an "official" answer).

To achieve your formatting objective, you should replace the third hfill directive with an all-blank line (to induce a paragraph break) followed by bigskip before the next instance of begin{tabular}.

Some additional comments: (i) You may (actually, should) omit the centering directive as it doesn't do anything in the your setup. (ii) The sixth and final hfill directive should be deleted (or commented out). (iii) bm (short for "bold math") should be employed only in math mode; hence, do replace bm{$K_0$} with $bm{K_0}$, bm{$J_0$} with $bm{J_0}$, etc. (iv) Using [H] to force the placement of the table "right here, damn it!" can actually be counterproductive; use [ht!] instead.

documentclass{scrartcl}
usepackage{float}
usepackage[a4paper, vmargin=3cm, left=2.5cm, right=2cm]{geometry} 
usepackage{bm}
begin{document}

begin{table}[H]
    %centering % <-- not needed
    begin{tabular}{c|cccc}
      $bm{K_0}$ & $Q_0$ & $Q_0$ & $bar Q_0$ & $bar Q_0$ 
      hline
      $Q_1$      & X & 1 & X & X 
      $bar Q_1$ & 1 & 1 & X & X 
      hline
      & $Q_2$ & $bar Q_2$ & $bar Q_2$ & $ Q_2$ 
    end{tabular}
    hfill
    begin{tabular}{c|cccc}
      $bm{J_0}$ & $Q_0$ & $Q_0$ & $bar Q_0$ & $bar Q_0$ 
      hline
      $Q_1$      & X & X & 1 & 0 
      $bar Q_1$ & X & X & 1 & 1 
      hline
      & $Q_2$ & $bar Q_2$ & $bar Q_2$ & $ Q_2$ 
    end{tabular}
    hfill
    begin{tabular}{c|cccc}
      $bm{K_1}$  & $Q_0$ & $Q_0$ & $bar Q_0$ & $bar Q_0$ 
      hline
      $Q_1$      & X & 1 & 0 & 1 
      $bar Q_1$ & X & X & X & X 
      hline
      & $Q_2$ & $bar Q_2$ & $bar Q_2$ & $ Q_2$ 
    end{tabular}

    bigskip % <-- new, in lieu of hfill
    begin{tabular}{c|cccc}
      $bm{J_1}$  & $Q_0$ & $Q_0$ & $bar Q_0$ & $bar Q_0$ 
      hline
      $Q_1$      & X & X & X & X 
      $bar Q_1$ & 1 & 1 & 0 & X 
      hline
      & $Q_2$ & $bar Q_2$ & $bar Q_2$ & $ Q_2$ 
    end{tabular}
    hfill
    begin{tabular}{c|cccc}
      $bm{K_2}$  & $Q_0$ & $Q_0$ & $bar Q_0$ & $bar Q_0$ 
      hline
      $Q_1$      & X & X & X & 1 
      $bar Q_1$ & 0 & X & 0 & X 
      hline
      & $Q_2$ & $bar Q_2$ & $bar Q_2$ & $ Q_2$ 
    end{tabular}
    hfill
    begin{tabular}{c|cccc}
      $bm{J_2}$  & $Q_0$ & $Q_0$ & $bar Q_0$ & $bar Q_0$ 
      hline
      $Q_1$      & X & 1 & X & X 
      $bar Q_1$ & X & X & 0 & X 
      hline 
      & $Q_2$ & $bar Q_2$ & $bar Q_2$ & $ Q_2$ 
    end{tabular}
    %hfill % <-- not needed either
    caption{Caption}
    label{tab:my_label}
end{table}
end{document}