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On Windows 7, using python 3.4.2 32 bits MSVCv.1600 or 3.4.1 64 bits, all the os.exec*e variants crash:
os.execle('C:/Python34/python.exe','Python.exe','-V',{})
os.execve('C:/Python34/python.exe',['python.exe','-V'],{})
os.execlpe('C:/Python34/python.exe','python.exe','-V',{})
os.execvpe('C:/Python34/python.exe',['python.exe','-V'],{})
Without any error message, windows will just open a "Python.exe has stopped working" window, be the scripts run from an interactive shell or invoking python script.py
On the other hand, 
os.execl('C:/Python34/python.exe','Python.exe','-V')
os.execve('C:/Python34/python.exe',['python.exe','-V'])
os.execlpe('C:/Python34/python.exe','python.exe','-V')
os.execvpe('C:/Python34/python.exe',['python.exe','-V'])
will work perfectly.

You should read:
os.execv('C:/Python34/python.exe',['python.exe','-V'])
os.execlp('C:/Python34/python.exe','python.exe','-V')
os.execvp('C:/Python34/python.exe',['python.exe','-V'])
for the "what works".

Calling nt.execv works with a unicode string because it creates a bytes path via PyUnicode_FSConverter. OTOH, nt.execve uses path_converter, which doesn't convert unicode to bytes on Windows. Thus in posixmodule.c, for the call execve(path->narrow, argvlist, envlist), path->narrow is NULL, and the CRT kills the process due to a bad argument:
 (70.135c): Break instruction exception - code 80000003 (first chance)
 ntdll!LdrpDoDebuggerBreak+0x30:
 00000000`7733cb70 cc int 3
 0:000> bp ntdll!ZwTerminateProcess
 0:000> bp desktopcrt140!execve
 0:000> g
 ModLoad: 000007fe`fc850000 000007fe`fc868000 C:\Windows\system32\CRYPTSP.dll
 ModLoad: 000007fe`fc550000 000007fe`fc597000 C:\Windows\system32\rsaenh.dll
 ModLoad: 000007fe`fceb0000 000007fe`fcebf000 C:\Windows\system32\CRYPTBASE.dll
 Python 3.5.0a1 (v3.5.0a1:5d4b6a57d5fd, Feb 7 2015, 18:15:14) 
 [MSC v.1900 64 bit (AMD64)] on win32
 Type "help", "copyright", "credits" or "license" for more information.
 >>> import nt, sys 
 >>> path = sys.executable
 >>> nt.execve(path, ['python', '-V'], {})
 Breakpoint 1 hit
 DESKTOPCRT140!_execve:
 000007fe`fa5ac43c 4d8bc8 mov r9,r8
 0:000> r rcx
 rcx=0000000000000000
The first argument is passed in AMD64 register rcx, which you can see is NULL here.
 0:000> g
 Breakpoint 0 hit
 ntdll!NtTerminateProcess:
 00000000`772e1570 4c8bd1 mov r10,rcx
 0:000> k 8
 Child-SP RetAddr Call Site
 00000000`0038f5c8 000007fe`fd11402f ntdll!NtTerminateProcess
 00000000`0038f5d0 000007fe`f54e003a KERNELBASE!TerminateProcess+0x2f
 00000000`0038f600 000007fe`f54e0055 APPCRT140!_invalid_parameter+0x76
 00000000`0038f640 000007fe`fa5abbd8 APPCRT140!_invalid_parameter_noinfo+0x19
 00000000`0038f680 00000000`6aa116a7 DESKTOPCRT140!common_spawnv<char>+0x44
 00000000`0038f6e0 00000000`6aa115a8 python35!os_execve_impl+0xb7
 00000000`0038f720 00000000`6aa9ff3b python35!os_execve+0xa8
 00000000`0038f7d0 00000000`6ab14aed python35!PyCFunction_Call+0xfb
Using a bytes path with nt.execve will work, but it's deprecated on Windows:
 C:\>py -3.5 -Wall
 Python 3.5.0a1 (v3.5.0a1:5d4b6a57d5fd, Feb 7 2015, 18:15:14) 
 [MSC v.1900 64 bit (AMD64)] on win32
 Type "help", "copyright", "credits" or "license" for more information.
 >>> import os, nt, sys
 >>> path = os.fsencode(sys.executable)
 >>> nt.execve(path, ['python', '-V'], {})
 __main__:1: DeprecationWarning: The Windows bytes API has been 
 deprecated, use Unicode filenames instead
 C:\>Python 3.5.0a1
Since bytes paths are deprecated on Windows, these calls should be using wexecv and wexecve.
https://msdn.microsoft.com/en-us/library/431x4c1w%28v=vs.100%29.aspx 

Just hit that bug. Is this really not yet fixed in any newer version?

> Just hit that bug. Is this really not yet fixed in any newer version?
The bug was fixed in Python 3.6.0. It was fixed by Steve Dower in its implementation of the PEP 529, in the change e20c7d8a8187:
- execv(path_char, argvlist);
+#ifdef HAVE_WEXECV
+ _wexecv(path->wide, argvlist);
+#else
+ execv(path->narrow, argvlist);
+#endif
I'm not sure that it's easy to backport the fix in Python 3.5.
Python 3.6 no walways use Unicode internally to call the Windows API, whereas Python 3.5 used bytes or Unicode depending on the arguments.
So I suggest to upgrade to Python 3.6, or use bytes in the meanwhile.
Would it be ok to close the bug?

It's straightforward to detect path->narrow == NULL and raise instead of crashing. We can also trivially check path->wide on Windows and raise with a more specific error.
Given it's already fixed for 3.6.0 and there's only one more binary release for 3.5 (and Windows has *very* limited uptake apart from the binary releases), I don't think it's worth making this work given how many versions have been broken in the past.