Late review:

What further functionality and documentation would a user expect to actually consider using this library?

Spelling

10+ spelling errors noted. Use an auto-spelling checker to help make a good 1st impression.

String compare

Note that strcmp(a, b) compares as if the strings were made of unsigned char even if char is signed. Make certain your function implementations follow that.

Name-space scattering

Code scatters usage across the name space risking collisions with other code.

Code defines:

STR_VIEW
str_view
LES
EQL
GRT
ERR
sv_threeway_cmp
SVLEN
SV
sv
sv_...(dozens of functions)

I recommend all these begin with sv, sv_ or SV_ including the file name.

field vs. member

"It consists of a pointer to const char data and a size_t field." --> C spec calls .s and .sz of str_view members.

Design

Its not a 3-way, but 4.

Rather than return 1 of 4 values for compares consider 1) returning 3 as int with -1, 0, 1 and a defined result for NULL arguments (recommended) or 2) return a 4-bit mask for each <, ==, >, incomparable.

The point of enumeration is abstraction. The -1,0,1,2 is trying to have apply meaning to the abstract. Choose one (enum 0,1,2,3) or the other (int).

Late review:

What further functionality and documentation would a user expect to actually consider using this library?

Spelling

10+ spelling errors noted. Use a spell checker to help make a good 1st impression.

String compare

Note that strcmp(a, b) compares as if the strings were made of unsigned char even if char is signed. Make certain your function implementations follow that.

Name-space scattering

Code scatters usage across the name space risking collisions with other code.

Code defines:

STR_VIEW
str_view
LES
EQL
GRT
ERR
sv_threeway_cmp
SVLEN
SV
sv
sv_...(dozens of functions)

I recommend all these begin with sv_ or SV_ (or is sv) including the file name.

field vs. member

"It consists of a pointer to const char data and a size_t field." --> C spec calls .s and .sz of str_view members.

Design

Its not a 3-way, but 4.

Rather than return 1 of 4 values for compares consider 1) returning 3 as int with -1, 0, 1 and a defined result for NULL arguments (recommended) or 2) return a 4-bit mask for each <, ==, >, incomparable.

The point of enumeration is abstraction. The -1,0,1,2 is trying to have apply meaning to the abstract. Choose one (enum 0,1,2,3) or the other (int).