What is the difference between קדד, כרע, and שׁחה?

Question

The DCH and BDB lexicons list these words as all meaning the same thing - "to bow down." Is there some finer nuance between these words?

Answer 1

The Peninei Halacha basing itself on two instances in Shas, writes about the Yomim Noraim (7:14) the following:

שלושה סוגים של השתחוויה ישנם: השתחוויה גמורה היא כשהגוף שטוח על הארץ בפישוט ידיים ורגליים. קידה היא שהאדם נותר עומד על רגליו וכופף ראשו עד שהוא מגיע לארץ. כריעה היא שאדם יורד על ברכיו וכופף את גופו עד שפניו מגיעות לארץ (ברכות לד, ב; שבועות טז, ב). המנהג הרווח ביום הכיפורים לכרוע, ויש שמשתחווים בפישוט ידיים ורגליים.

There are three types of bowing: 1) (שחה) A complete prostration is when the body is flat on the ground with one’s hands and legs spread. 2) (קדה) Kidda is that the person remains standing on his feet and bends his head until he reaches the ground. 3) (כרע) is a person getting down on his knees and bending his body until his face reaches the ground (see Berachos 34b & Shevuous 16b).The prevailing custom on Yom Kippur is to kneel (כרע), and some bow (שחה) stretching ones hands and feet

קידה involves bowing to the ground and is a somewhat more flexible/contortionist a movement that is apparently very rare for one to be able to do. The Gemara in Sukkah 53a recalls how Rabbi Shimon ben Gamliel would rejoice at the Simchas Beis Hashoeivah, how he would juggle fire and perform a sort of bow whilst doing so. It writes there:

תַּנְיָא: אָמְרוּ עָלָיו עַל רַבָּן שִׁמְעוֹן בֶּן גַּמְלִיאֵל, כְּשֶׁהָיָה שָׂמֵחַ שִׂמְחַת בֵּית הַשּׁוֹאֵבָה, הָיָה נוֹטֵל שְׁמֹנֶה אֲבוּקוֹת שֶׁל אוֹר, וְזוֹרֵק אַחַת וְנוֹטֵל אַחַת וְאֵין נוֹגְעוֹת זוֹ בָּזוֹ. וּכְשֶׁהוּא מִשְׁתַּחֲוֶה, נוֹעֵץ שְׁנֵי גּוּדָלָיו בָּאָרֶץ וְשׁוֹחֶה וְנוֹשֵׁק אֶת הָרִצְפָּה וְזוֹקֵף, וְאֵין כׇּל בְּרִיָּה יְכוֹלָה לַעֲשׂוֹת כֵּן, וְזוֹ הִיא קִידָּה.

It is taught in a baraita: They said about Rabban Shimon ben Gamliel that when he would rejoice at the Celebration of the Place of the Drawing of the Water, he would take eight flaming torches and toss one and catch another, juggling them, and, though all were in the air at the same time, they would not touch each other. And when he would prostrate himself, he would insert his two thumbs into the ground, and bow, and kiss the floor of the courtyard and straighten, and there was not any other creature that could do that due to the extreme difficulty involved. And this was the form of bowing called kidda performed by the High Priest. (Sefaria translation and notation)

Rashi there adds:

וזו קידה - האמורה בכתובים דאמר מר קידה על אפים אין לו להשתטח להגיע לארץ גופו אלא פניו בלבד מי שיודע ויכול לעשות כן ובדורו של רבן שמעון לא היה אחד מעומדי עזרה יכול לעשות כן אלא הוא:

This is Kiddah - it is spoken about in writings that Mar said Kiddah is bowing upon one's face, meaning that one doesn't need to bend to reach the ground with his body but only his face alone, and only Rabbi Shimon knew and was able to do this in his generation, no one else from those that stood in the Temple courtyard was able to do this other than him.

Answer 2

These are all forms of bowing: שחה means to bow from the hips while standing, כרע means to bow on to knees not neccessarily with the head down, קד (קדה) means to be on the ground on toes and thumbs only and getting up at once onto the feet (do not ever try that you will hurt and even cripple yourself severly!!).