A noun translated “crow” (SD/426). Tolkien listed this noun in two forms, khâu and khô, both as examples of seemingly uniconsonantal nouns that prehistorically were biconsonantal (from prehistoric ✶khaw). Most authors have suggested these are simply variations on the same noun illustrating different paths of phonetic development (AAD/18, AL/Adûnaic). Another interesting possibility is that khâu is an (archaic?) subjective form of khô, produced from the usual a-fortification of primitive ✶khaw → ✶khāw, which would develop phonetically in Classical Adûnaic to khâu and khô. As support for this idea, the plural form of khâu is given as khāwī(m), which appears to include the subjective plural suffix -im.
This line of reasoning is quite speculative. Nevertheless, it is probably easier to use the form khô over khâu, since the inflections of khô would be more regular: plural khôi, dual ✱khôwat, objective ✱khôwu, etc.
This root first appeared as ᴹ√SED “rest” in The Etymologies of the 1930s with derivatives like ᴹQ. sére “rest, repose, peace” and N. sîdh “peace”, along with the name of the Valië ᴹQ. Este < ᴹ✶ezdē (Ety/SED). The root √SED “rest” along with the etymology of Este reappeared in both the Outline of Phonetic Development (OP1) from the 1940s (PE19/45) and the Outline of Phonology (OP2) from the early 1950s (PE19/92), and again in revisions to the OP2 page annotated in green ink and thus probably from 1970 (PE19/91 and note #110).
The root √SED “rest” was mentioned a couple times in the Quenya Verbal System (QVS) from 1948 as the basis for the verb ᴹQ. ser- “rest, repose” (PE22/102, 125) and again (unglossed) in the Quendi and Eldar essay of 1959-60 as the basis for Este (WJ/403). In the sense “peace” this root had some competition, however. In the first version of the Gloria in Excelsis Deo prayer from the mid-1960s, Tolkien used Q. sívë for “peace”, revised to Q. sérë in the second version and then Q. rainë (VT44/32). The word Q. sívë “peace” reappeared in unpublished etymological notes from 1969 derived from the root √SIB “rest, quiet” (VT44/35).
Neo-Eldarin: For purposes of Neo-Eldarin, I would use √SED for the act of resting and so Q. sérë “peace” = “restfulness (personal peace)”, and would use √SIB = “quiet, ✱stillness, absence of activity” and so Q. sívë “peace” = “absence of activity or violence (environmental peace)”. I would use Q. rainë as “good will” in connection to √RAY “smile”.